<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article  PUBLIC "-//NLM//DTD Journal Publishing DTD v3.0 20080202//EN" "http://dtd.nlm.nih.gov/publishing/3.0/journalpublishing3.dtd"><article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" dtd-version="3.0" xml:lang="en" article-type="research article"><front><journal-meta><journal-id journal-id-type="publisher-id">AiM</journal-id><journal-title-group><journal-title>Advances in Microbiology</journal-title></journal-title-group><issn pub-type="epub">2165-3402</issn><publisher><publisher-name>Scientific Research Publishing</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="doi">10.4236/aim.2019.93012</article-id><article-id pub-id-type="publisher-id">AiM-90924</article-id><article-categories><subj-group subj-group-type="heading"><subject>Articles</subject></subj-group><subj-group subj-group-type="Discipline-v2"><subject>Biomedical&amp;Life Sciences</subject></subj-group></article-categories><title-group><article-title>
 
 
  Analysis of the Alkane Hydroxylase Gene and Long-Chain Cyclic Alkane Degradation in &lt;i&gt;Rhodococcus&lt;/i&gt;
 
</article-title></title-group><contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Taiki</surname><given-names>Kawagoe</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Kenzo</surname><given-names>Kubota</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Kiwako</surname><given-names>S. Araki</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref><xref ref-type="corresp" rid="cor1"><sup>*</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Motoki</surname><given-names>Kubo</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib></contrib-group><aff id="aff1"><addr-line>Department of Biotechnology, Faculty of Life Sciences, Ritsumeikan University, Shiga, Japan</addr-line></aff><pub-date pub-type="epub"><day>04</day><month>03</month><year>2019</year></pub-date><volume>09</volume><issue>03</issue><fpage>151</fpage><lpage>163</lpage><history><date date-type="received"><day>30,</day>	<month>January</month>	<year>2019</year></date><date date-type="rev-recd"><day>2,</day>	<month>March</month>	<year>2019</year>	</date><date date-type="accepted"><day>5,</day>	<month>March</month>	<year>2019</year></date></history><permissions><copyright-statement>&#169; Copyright  2014 by authors and Scientific Research Publishing Inc. </copyright-statement><copyright-year>2014</copyright-year><license><license-p>This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/</license-p></license></permissions><abstract><p>
 
 
  To characterize the long-chain cyclic alkane (
  c-alkane) degradation of bacteria in 
  Rhodococcus, we analyzed the relationship between the alkane hydroxylase gene (
  alkB) and long-chain 
  c-alkane degradation in 19 species. Eleven strains which were isolated from nature using long-chain 
  c-alkane as a substrate were identified as 
  R. erythropolisc, and all were shown to carry the 
  alkB [
  alkB R2 type]. This gene type was also carried by two other species, 
  R. rhodochrous and 
  R. baikonurensis. In total, 17 species of the genus 
  Rhodococcus carried 
  alkB, but the gene types differed from each other. The two species 
  R. rhodnii and 
  R. coprophilus did not carry 
  alkB, and their long-chain 
  c-alkane degradation levels were low.
 
</p></abstract><kwd-group><kwd>Alkane Hydroxylase</kwd><kwd> &lt;i&gt;alkB&lt;/i&gt;</kwd><kwd> Bioremediation</kwd><kwd> Long-Chain &lt;i&gt;c&lt;/i&gt;-Alkane</kwd><kwd> &lt;i&gt;Rhodococcus&lt;/i&gt;</kwd></kwd-group></article-meta></front><body><sec id="s1"><title>1. Introduction</title><p>In recent years, environmental pollution by petroleum hydrocarbons has been a serious problem worldwide. To clean up hydrocarbon-contaminated sites, a range of treatments can be performed, including incineration, chemical oxidation (Fenton reaction), washing, and evaporation. However, a lot of energy is required for these treatments [<xref ref-type="bibr" rid="scirp.90924-ref1">1</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref2">2</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref3">3</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref4">4</xref>]. Biological treatments such as bioremediation have been investigated as an effective system of hydrocarbon degradation [<xref ref-type="bibr" rid="scirp.90924-ref5">5</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref6">6</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref7">7</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref8">8</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref9">9</xref>] , and many microorganisms with the ability to degrade petroleum hydrocarbons have been isolated from their natural habitat and characterized [<xref ref-type="bibr" rid="scirp.90924-ref10">10</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref11">11</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref12">12</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref13">13</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref14">14</xref>]. However, it is difficult for bacteria to degrade long-chain hydrocarbons, especially long-chain cyclic alkanes (c-alkanes) [<xref ref-type="bibr" rid="scirp.90924-ref15">15</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref16">16</xref>]. Therefore, the isolation of bacteria capable of degrading long-chain hydrocarbons is important for the bioremediation of hydrocarbons [<xref ref-type="bibr" rid="scirp.90924-ref17">17</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref18">18</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref19">19</xref>].</p><p>Kubota et al. previously isolated hydrocarbon-degrading bacteria (HDB) from their natural habitats, and determined their phylogenetic relationships based on partial sequences of the 16S rRNA gene [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>]. This identified many isolated species as belonging to the genera Pseudomonas, Rhodococcus, Gordonia, and Acinetobacter [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>]. The strains belonging to the genera Rhodococcus and Gordonia degraded not only long-chain normal alkanes (n-alkanes) but also c-alkanes as the sole carbon and energy source [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref21">21</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref22">22</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref23">23</xref>]. This indicated that the strains degrading long-chain c-alkanes have potential for cleaning petroleum hydrocarbon pollutants, because long-chain n-alkanes and c-alkanes remain in the soil for long periods.</p><p>The first step of degrading long-chain c-alkanes, the oxidation of hydrocarbons, is catalyzed by alkane hydroxylases (Alk) [<xref ref-type="bibr" rid="scirp.90924-ref24">24</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref25">25</xref>]. For many HDB, alkB genes encoding Alk have been well analyzed [<xref ref-type="bibr" rid="scirp.90924-ref26">26</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref27">27</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref28">28</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref29">29</xref>]. alkB genes are categorized as seven types (alkB1 to alkB7) [<xref ref-type="bibr" rid="scirp.90924-ref30">30</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref31">31</xref>] , and many strains of the genus Rhodococcus are known to carry alkB2. However, the relationship between different types of alk genes and the c-alkane degradation ability has not been characterized for each species. Moreover, alkB varies among species of the genus Rhodococcus, and oxidizable substrates differ depending on the alkB gene type [<xref ref-type="bibr" rid="scirp.90924-ref32">32</xref>]. For example, Fukuhara et al. [<xref ref-type="bibr" rid="scirp.90924-ref33">33</xref>] revealed that the R. erythropolis strain NDKK6, carrying the alkB R2 type, showed a high c-alkane degradation ability. Furthermore, the carbon number of the alkyl side chain seemed to influence c-alkane degradation [<xref ref-type="bibr" rid="scirp.90924-ref34">34</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref35">35</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref36">36</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref37">37</xref>]. A previous study isolated 11 strains of HDB from the genus Rhodococcus [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>] , but they have not been characterized yet.</p><p>In the present study, we investigated the relationship between alkB genes and long-chain c-alkane degradation for 11 isolated strains of Rhodococcus and 19 strains obtained from the NITE Biological Resource Center (NBRC), Japan.</p></sec><sec id="s2"><title>2. Materials and Methods</title><sec id="s2_1"><title>2.1. Bacterial Strains</title><p>Thirty bacterial strains were used as long-chain c-alkane-degrading bacteria. Eleven strains (R. erythropolis NDKK1, R. erythropolis NDKK2, R. erythropolis NDKK5, R. erythropolis NDKK6, R. erythropolis NDKK7, R. erythropolis NDKK48, R. erythropolis ODNM1C, R. erythropolis NDKY82A, R. erythropolis ODMI54, R. erythropolis ODNM2B, and R. erythropolis NDMI144) were isolated in our previous study [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>] , and 19 species (R. erythropolis NBRC15567, R. rhodochrous NBRC15564, R. baikonurensis NBRC100611, R. wratislaviensis NBRC100605, R. opacus NBRC100624, R. ruber NBRC15591, R. equi NBRC101255, R. percoletus NBRC100626, R. jostii NBRC16295, R. triatomae NBRC103116, R. koreensis NBRC100607, R. corynebacterioides NBRC14404, R. zopfii NBRC100606, R. tukisamuensis NBRC100609, R. maanshanensis NBRC100610, R. pyridinivorans NBRC100608, R. kroppenstedtii NBRC103113, R. rhodnii NBRC100604, and R. coprophilus NBRC100603) were obtained from the NBRC.</p></sec><sec id="s2_2"><title>2.2. Identification of Bacterial Strains</title><p>The 11 strains from our earlier study were identified based on the full length 16S rRNA gene sequence. These strains were cultivated in Luria-Bertani (LB) medium (1% peptone, 0.5% yeast extract, and 0.5% NaCl) at 30˚C with shaking at 200 rpm [<xref ref-type="bibr" rid="scirp.90924-ref36">36</xref>]. Total DNA was extracted from the culture, and the 16S rRNA gene sequence was determined [<xref ref-type="bibr" rid="scirp.90924-ref38">38</xref>] [<xref ref-type="bibr" rid="scirp.90924-ref39">39</xref>]. Sequence data were deposited in the DNA Data Bank of Japan (DDBJ) database (https://www.ddbj.nig.ac.jp/index-e.html) under accession numbers LC107434 to LC107444. The sequences were also compared with data in the GenBank database using BLAST+ 2.2.31 which is available at https://www.ncbi.nlm.nih.gov/blast/. Phylogenetic analyses were carried out for the 11 strains and the additional 19 species whose sequence data were obtained from the GenBank database using ClustalW (version 2.1). The phylogenetic tree was constructed by the neighbor-joining method and bootstrap values were calculated by 1000 replications.</p></sec><sec id="s2_3"><title>2.3. Sequences of alkB and alkB [alkB R2 Type] Genes</title><p>Sequences of alkB were also determined for 30 strains. PCR was conducted using primers alkB-F (5’-AACTAYMTCGARCAYTAYGG-3’)/alkB-R (5’-TGRTCKSTCGYTGVARGTG-3’) and alkB R2-F (5’-CGGTTGTGTCGCAGGA-TC-3)/ alkB R2-R (5’-AACGACTGCGCCAGAGTGAT-3’) [<xref ref-type="bibr" rid="scirp.90924-ref31">31</xref>] to confirm the presence of alkB and alkB genes [alkB R2 type], respectively. alkB and alkB gene [alkB R2 type] PCR products were 140 bp and 100 bp, respectively. alkB primers were designed for the consensus region of HDB. alkB sequences were determined by the ABI PRISM 3100 Genetic Analyzer (Applied Biosystems, Foster City, CA, USA), and sequence data were deposited in the DDBJ database under accession numbers LC107445 to LC107470. Phylogenetic analysis was performed for alkB sequences as described above.</p></sec><sec id="s2_4"><title>2.4. Analysis of c-Alkane Degradation by Rhodococcus Strains</title><p>To evaluate bacterial long-chain c-alkane degrading abilities, 30 strains were cultivated in modified SW medium with undecylcyclohexane (UDC), dodecylcyclohexane (DDC), and tridecylcyclohexane (TDC) substrates. Pre-culture of each strain was prepared in LB medium. Substrate hydrocarbon (0.10 g) and pre-culture (1 ml) were inoculated in 100 ml of modified SW medium (1% v/w; per liter: 1.21 g NH<sub>4</sub>NO<sub>3</sub>, 14.3 g Na<sub>2</sub>HPO<sub>4</sub>・12H<sub>2</sub>O, 5.44 g KH<sub>2</sub>PO<sub>4</sub>, 0.5 g NaCl, 0.247 g MgSO<sub>4</sub>, 2.78 mg FeSO<sub>4</sub>・7H<sub>2</sub>O, 14.7 mg CaCl<sub>2</sub>・2H<sub>2</sub>O, 2.01 mg ZnSO<sub>4</sub>∙7H<sub>2</sub>O, 0.15 mg [NH<sub>4</sub>]<sub>6</sub>Mo<sub>7</sub>O<sub>24</sub>・4H<sub>2</sub>O, 2 mg CuSO<sub>4</sub>・5H<sub>2</sub>O, 0.4 mg CoCl<sub>2</sub>・6H<sub>2</sub>O, 1.49 mg MnSO<sub>4</sub>・5H<sub>2</sub>O, 0.5 g polypeptone, and 0.25 g yeast extract) in a baffle flask before incubating at 30˚C with shaking at 120 rpm.</p><p>After 3 days of cultivation, 30 ml of chloroform-methanol (3:1) mixture was added to 100 ml medium and mixed well before centrifuging at 4000&#215; g for 30 min. The separated organic layer was taken for gas-chromatographic analysis using a flame ionized detector (GC-FID) (GC-2010, Shimadzu, Kyoto, Japan). The degradation test was carried out in triplicate. The degradation ratio was calculated from the rate of decrease of the peak area of the gas-chromatogram as follows:</p><p>1 − (Peak area after cultivation/Peak area before cultivation) &#215; 100 (%)</p><p>The concentration of hydrocarbon in the soil was measured using the OCMA-355 oil content analyzer (Horiba, Kyoto, Japan).</p></sec></sec><sec id="s3"><title>3. Results and Discussion</title><sec id="s3_1"><title>3.1. Identification of Long Chain c-Alkane-Degrading Bacteria and Analysis of Alkane Hydroxylase Genes in Rhodococcus</title><p>To identify Rhodococcus strains isolated in our previous study [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>] , we determined the 16S rRNA gene sequences (1429 - 1472 bp). Phylogenetic analysis showed that all strains belonged to R. erythropolis, based on similarity with the reference strain R. erythropolis MPU33 (DNA databank accession number: AB334770) (<xref ref-type="fig" rid="fig1">Figure 1</xref>). These strains were isolated from a culture medium containing long chain c-alkane as the sole carbon source [<xref ref-type="bibr" rid="scirp.90924-ref20">20</xref>] , indicating that the strains carried alkB.</p><p>The presence and type of alkB genes in the strains were next examined. All strains carried alkB genes, with 13 Rhodococcus species of the 29 strains used in this study shown to carry alkB (<xref ref-type="table" rid="table1">Table 1</xref> and <xref ref-type="table" rid="table2">Table 2</xref>). alkB sequences of R. erythropolis isolated from nature were 98.1% - 100% homologous with the alkB R2 type of R. erythropolis NDKK6 that has high c-alkane degradation ability (<xref ref-type="table" rid="table1">Table 1</xref>). The alkB sequence from R. erythropolis NBRC15567 also showed high similarity with alkB R2 type (95.7%). Additionally, two other strains (R. rhodochrous and R. baikonurensis) obtained from NBRC carried alkB genes with approximately 100% homology to alkB R2 type (<xref ref-type="table" rid="table2">Table 2</xref>).</p><p>Ten strains also carried alkB genes, but homology analysis showed that their sequences differed from alkB R2 type (21.9% to 78.3% homologous) so original alkB names were given to each alkB gene type (<xref ref-type="table" rid="table2">Table 2</xref>). The similarity of R. opacus and R. percolatus alkB genes was 99.7% (<xref ref-type="fig" rid="fig2">Figure 2</xref>), so they were named alkB ROP type. The remaining 6 species lacked alkB genes. Phylogenetic analysis based on alkB revealed separate clusters of alkB [alkB R2 type] and alkB [non-alkB R2 type] groups (<xref ref-type="fig" rid="fig2">Figure 2</xref>). In the Rhodococcus genus, 3 groups were categorized as follows: first group included species such as R. baikonurensis and R. rhodochrous carrying alkB R2 type, another group carried other types of alkB genes, and the third group lacked any alkB genes.</p></sec><sec id="s3_2"><title>3.2. Degradation of Long Chain c-Alkane by Rhodococcus Species Carrying alkB R2 Type</title><p>The degradation rates of long chain c-alkanes (UDC, DDC, and TDC) by 14</p><table-wrap id="table1" ><label><xref ref-type="table" rid="table1">Table 1</xref></label><caption><title> The presence of alkB genes and similarity to alkB [alkB R2 type] genes</title></caption><table><tbody><thead><tr><th align="center" valign="middle" >Strain</th><th align="center" valign="middle" >alkB Gene</th><th align="center" valign="middle" >Similarity to alkB Gene [alkB R2 Type] (%)*</th></tr></thead><tr><td align="center" valign="middle" >R. erythropolis NDKK1</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.6</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK2</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >100</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK5</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.9</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK7</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.8</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK48</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.7</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODNM1C</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.8</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKY82A</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >98.1</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODMI54</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.7</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODNM2B</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.2</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDMI144</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.7</td></tr><tr><td align="center" valign="middle" >R. erythropolis NBRC15567</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >95.7</td></tr></tbody></table></table-wrap><p>*alkB gene [alkB R2 type] from Rhodococcus erythropolis NDKK6 [<xref ref-type="bibr" rid="scirp.90924-ref1">1</xref>].</p><table-wrap id="table2" ><label><xref ref-type="table" rid="table2">Table 2</xref></label><caption><title> The presence of alkB genes similarity to alkB genes [alkB R2 type], and types of alkB gene</title></caption><table><tbody><thead><tr><th align="center" valign="middle" >Strain</th><th align="center" valign="middle" >alkB Gene</th><th align="center" valign="middle" >Similarity to alkB Gene [alkB R2 Type] (%)*</th><th align="center" valign="middle" >Type of alkB gene</th></tr></thead><tr><td align="center" valign="middle" >R. rhodochrous NBRC15564</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >100</td><td align="center" valign="middle" >alkB R2</td></tr><tr><td align="center" valign="middle" >R. baikonurensis NBRC100611</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >99.7</td><td align="center" valign="middle" >alkB R2</td></tr><tr><td align="center" valign="middle" >R. wratislaviensis NBRC100605</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >22.6</td><td align="center" valign="middle" >alkB RW</td></tr><tr><td align="center" valign="middle" >R. opacus NBRC100624</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >23.4</td><td align="center" valign="middle" >alkB ROP</td></tr><tr><td align="center" valign="middle" >R. ruber NBRC15591</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >30.5</td><td align="center" valign="middle" >alkB RR</td></tr><tr><td align="center" valign="middle" >R. equi NBRC101255</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >78.3</td><td align="center" valign="middle" >alkB RE</td></tr><tr><td align="center" valign="middle" >R. percolatus NBRC100626</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >21.9</td><td align="center" valign="middle" >alkB ROP</td></tr><tr><td align="center" valign="middle" >R. jostii NBRC16295</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >30.1</td><td align="center" valign="middle" >alkB RJ</td></tr><tr><td align="center" valign="middle" >R. triatomae NBRC103116</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >30.1</td><td align="center" valign="middle" >alkB R1</td></tr><tr><td align="center" valign="middle" >R. koreensis NBRC100607</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >30.5</td><td align="center" valign="middle" >alkB RK</td></tr><tr><td align="center" valign="middle" >R. corynebacterioides NBRC14404</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >24.1</td><td align="center" valign="middle" >alkB RC</td></tr><tr><td align="center" valign="middle" >R. zopfii NBRC100606</td><td align="center" valign="middle" >+</td><td align="center" valign="middle" >30.7</td><td align="center" valign="middle" >alkB RZ</td></tr><tr><td align="center" valign="middle" >R. tukisamuensis NBRC100609</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr><tr><td align="center" valign="middle" >R. maanshanensis NBRC100610</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr><tr><td align="center" valign="middle" >R. pyridinivorans NBRC100608</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr><tr><td align="center" valign="middle" >R. kroppenstedtii NBRC103113</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr><tr><td align="center" valign="middle" >R. rhodnii NBRC100604</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr><tr><td align="center" valign="middle" >R. coprophilus NBRC100603</td><td align="center" valign="middle" >−</td><td align="center" valign="middle" ></td><td align="center" valign="middle" ></td></tr></tbody></table></table-wrap><p>*alkB gene [alkB R2 type] from Rhodococcus erythropolis NDKK6 [<xref ref-type="bibr" rid="scirp.90924-ref1">1</xref>].</p><p>Rhodococcus strains carrying alkB R2 type are shown in <xref ref-type="table" rid="table3">Table 3</xref>. The average degradation rates of UDC, DDC, and TDC were 55.5%, 31.8%, and 55.2%, respectively. Strain ODNM2B showed the highest degradation rate (98.9%) of UDC. Similarly, strains NDKK2 and ODNM2B showed superior degradation of DDC and TDC (42.9% and 94.6%, respectively). <xref ref-type="fig" rid="fig3">Figure 3</xref> shows the degradation of UDC by R. erythropolis NDKK6. These observations of R. erythropolis strains including our identified strains and one from NBRC indicate that alkB R2 type plays an important role in the degradation of long chain c-alkanes, and that Rhodococcus carrying alkB R2 type can utilize n-hexadecane [<xref ref-type="bibr" rid="scirp.90924-ref40">40</xref>].</p></sec><sec id="s3_3"><title>3.3. Degradation of Long Chain c-Alkane by Strains Carrying Non-alkB R2 Type</title><p>The degradation rates of long chain c-alkanes (UDC, DDC, and TDC) by 10 Rhodococcus species carrying non-alkB R2 type are shown in <xref ref-type="table" rid="table4">Table 4</xref>. Average degradation rates of UDC, DDC, and TDC were 49.4%, 29.4%, and 53.9%, respectively. Although strains of the genus Rhodococcus carrying alkB genes can degrade hydrocarbons such as long chain c-alkanes, the degradation rates of strains carrying non-alkB R2 type were lower than those carrying alkB R2 type</p><p>(<xref ref-type="table" rid="table3">Table 3</xref> and <xref ref-type="table" rid="table4">Table 4</xref>). This indicates that alkB R2 type was more suitable for the degradation of long chain c-alkanes. Moreover, the higher degradation rates of UDC and TDC compared with DDC suggest that long chain c-alkanes with an odd number of carbon atoms are easier to biodegrade by Rhodococcus species [<xref ref-type="bibr" rid="scirp.90924-ref21">21</xref>].</p></sec><sec id="s3_4"><title>3.4. Degradation of Long Chain c-Alkane by Strains Carrying No alkB Genes</title><p>The degradation rates of long chain c-alkanes (UDC, DDC, and TDC) by 6 strains carrying no alkB genes are shown in <xref ref-type="table" rid="table5">Table 5</xref>. R. rhodnii NBRC100604 had the highest degradation rate of UDC, DDC, and TDC among all 6 strains. The average degradation rates of UDC, DDC, and TDC by these 6 strains were 19.3%, 13.3%, and 19.1%, respectively. The average degradation ratios of UDC, DDC, and TDC by strains carrying no alkB genes were lower than in these by strains carrying alkB [alkB R2 type] and alkB [non-alkB R2 type]. Strains of genus Rhodococcus carrying alkB genes had higher degradation abilities of long-chain c-alkane than those of strains carrying no alkB genes (<xref ref-type="table" rid="table3">Table 3</xref> and <xref ref-type="table" rid="table5">Table 5</xref>). However, R. rhodnii NBRC100604 carrying no alkB gene showed a degradation rate of above 30%. The ability of long-chain c-alkane degradation might be regulated</p><table-wrap id="table3" ><label><xref ref-type="table" rid="table3">Table 3</xref></label><caption><title> The degradation of long-chain c-alkanes by stains harboring alkB [alkB R2 type] in the genus Rhodococcus</title></caption><table><tbody><thead><tr><th align="center" valign="middle"  rowspan="2"  >Strain</th><th align="center" valign="middle"  colspan="3"  >Degradation Ratio (%)*</th></tr></thead><tr><td align="center" valign="middle" >Undecylcyclohexane (UDC)</td><td align="center" valign="middle" >Dodecylcyclohexane (DDC)</td><td align="center" valign="middle" >Tridecylcyclohexane (TDC)</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK1</td><td align="center" valign="middle" >16.0 &#177; 7.5</td><td align="center" valign="middle" >31.7 &#177; 8.7</td><td align="center" valign="middle" >17.9 &#177; 7.3</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK2</td><td align="center" valign="middle" >6.7 &#177; 10.7</td><td align="center" valign="middle" >42.9 &#177; 10.2</td><td align="center" valign="middle" >23.5 &#177; 3.2</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK5</td><td align="center" valign="middle" >66.7 &#177; 0.2</td><td align="center" valign="middle" >42.2 &#177; 1.8</td><td align="center" valign="middle" >40.6 &#177; 10.0</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK6</td><td align="center" valign="middle" >38.8 &#177; 1.0</td><td align="center" valign="middle" >18.9 &#177; 6.2</td><td align="center" valign="middle" >75.7 &#177; 6.1</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK7</td><td align="center" valign="middle" >53.5 &#177; 3.6</td><td align="center" valign="middle" >42.1 &#177; 4.0</td><td align="center" valign="middle" >46.7 &#177; 9.1</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKK48</td><td align="center" valign="middle" >40.4 &#177; 2.5</td><td align="center" valign="middle" >33.1 &#177; 12.6</td><td align="center" valign="middle" >81.4 &#177; 0.1</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODNM1C</td><td align="center" valign="middle" >78.2 &#177; 2.4</td><td align="center" valign="middle" >41.8 &#177; 2.9</td><td align="center" valign="middle" >91.0 &#177; 1.1</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDKY82A</td><td align="center" valign="middle" >52.7 &#177; 4.9</td><td align="center" valign="middle" >39.1 &#177; 9.2</td><td align="center" valign="middle" >78.4 &#177; 1.5</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODMI54</td><td align="center" valign="middle" >68.2 &#177; 5.0</td><td align="center" valign="middle" >37.8 &#177; 8.2</td><td align="center" valign="middle" >46.8 &#177; 2.6</td></tr><tr><td align="center" valign="middle" >R. erythropolis ODNM2B</td><td align="center" valign="middle" >98.9 &#177; 0.2</td><td align="center" valign="middle" >30.3 &#177; 3.5</td><td align="center" valign="middle" >94.6 &#177; 6.7</td></tr><tr><td align="center" valign="middle" >R. erythropolis NDMI144</td><td align="center" valign="middle" >91.4 &#177; 3.9</td><td align="center" valign="middle" >27.7 &#177; 10.3</td><td align="center" valign="middle" >39.9 &#177; 8.3</td></tr><tr><td align="center" valign="middle" >R. erythropolis NBRC15567</td><td align="center" valign="middle" >64.5 &#177; 9.4</td><td align="center" valign="middle" >0.0 &#177; 1.5</td><td align="center" valign="middle" >75.4 &#177; 1.5</td></tr><tr><td align="center" valign="middle" >R. erythropolis NBRC15564</td><td align="center" valign="middle" >50.6 &#177; 3.9</td><td align="center" valign="middle" >41.6 &#177; 9.1</td><td align="center" valign="middle" >39.6 &#177; 7.1</td></tr><tr><td align="center" valign="middle" >R. baikonurensis NBRC100611</td><td align="center" valign="middle" >50.4 &#177; 3.8</td><td align="center" valign="middle" >16.0 &#177; 3.0</td><td align="center" valign="middle" >20.6 &#177; 5.7</td></tr><tr><td align="center" valign="middle" >Average</td><td align="center" valign="middle" >55.5</td><td align="center" valign="middle" >31.8</td><td align="center" valign="middle" >55.2</td></tr></tbody></table></table-wrap><p>*n = 3.</p><table-wrap id="table4" ><label><xref ref-type="table" rid="table4">Table 4</xref></label><caption><title> The degradation of long-chain c-alkanes by stains harboring alkB genes [non-alkB R2 type] in the genus Rhodococcus</title></caption><table><tbody><thead><tr><th align="center" valign="middle"  rowspan="2"  >Strain</th><th align="center" valign="middle"  rowspan="2"  >Type of alkB Gene</th><th align="center" valign="middle"  colspan="3"  >Degradation Ratio (%)*</th></tr></thead><tr><td align="center" valign="middle" >Undecylcyclohexane (UDC)</td><td align="center" valign="middle" >Dodecylcyclohexane (UDC)</td><td align="center" valign="middle" >Tridecylcyclo-hexane (UDC)</td></tr><tr><td align="center" valign="middle" >R. wratislaviensis NBRC100605</td><td align="center" valign="middle" >alkB RW</td><td align="center" valign="middle" >39.7 &#177; 5.2</td><td align="center" valign="middle" >58.6 &#177; 4.0</td><td align="center" valign="middle" >25.1 &#177; 2.1</td></tr><tr><td align="center" valign="middle" >R. opacus NBRC100624</td><td align="center" valign="middle" >alkB ROP</td><td align="center" valign="middle" >94.9 &#177; 1.4</td><td align="center" valign="middle" >56.8 &#177; 5.8</td><td align="center" valign="middle" >43.1 &#177; 0.4</td></tr><tr><td align="center" valign="middle" >R. percoletus NBRC100626</td><td align="center" valign="middle" >alkB ROP</td><td align="center" valign="middle" >47.1&#177;13.5</td><td align="center" valign="middle" >35.7 &#177; 2.3</td><td align="center" valign="middle" >60.6 &#177; 11.1</td></tr><tr><td align="center" valign="middle" >R. ruber NBRC15591</td><td align="center" valign="middle" >alkB RR</td><td align="center" valign="middle" >98.2 &#177; 1.6</td><td align="center" valign="middle" >55.9 &#177; 3.6</td><td align="center" valign="middle" >43.2 &#177; 3.4</td></tr><tr><td align="center" valign="middle" >R. equi NBRC101255</td><td align="center" valign="middle" >alkB RE</td><td align="center" valign="middle" >48.9 &#177; 2.1</td><td align="center" valign="middle" >39.3 &#177; 3.4</td><td align="center" valign="middle" >98.4 &#177; 1.6</td></tr><tr><td align="center" valign="middle" >R. jostii NBRC16295</td><td align="center" valign="middle" >alkB RJ</td><td align="center" valign="middle" >29.8 &#177; 4.9</td><td align="center" valign="middle" >27.8 &#177; 10.7</td><td align="center" valign="middle" >65.0 &#177; 8.2</td></tr><tr><td align="center" valign="middle" >R. triatomae NBRC103116</td><td align="center" valign="middle" >alkB R1</td><td align="center" valign="middle" >34.0 &#177; 14.2</td><td align="center" valign="middle" >15.9 &#177; 7.4</td><td align="center" valign="middle" >75.3 &#177; 1.7</td></tr><tr><td align="center" valign="middle" >R. koreensis NBRC100607</td><td align="center" valign="middle" >alkB RK</td><td align="center" valign="middle" >6.0 &#177; 10.6</td><td align="center" valign="middle" >4.2 &#177; 5.4</td><td align="center" valign="middle" >5.5 &#177; 2.3</td></tr><tr><td align="center" valign="middle" >R. corynebacterioides NBRC14404</td><td align="center" valign="middle" >alkB RC</td><td align="center" valign="middle" >41.4 &#177; 6.5</td><td align="center" valign="middle" >0.0 &#177; 10.2</td><td align="center" valign="middle" >62.3 &#177; 0.6</td></tr><tr><td align="center" valign="middle" >R. zopfii NBRC100606</td><td align="center" valign="middle" >alkB RZ</td><td align="center" valign="middle" >53.9 &#177; 9.1</td><td align="center" valign="middle" >0.0 &#177; 9.7</td><td align="center" valign="middle" >60.3 &#177; 5.3</td></tr><tr><td align="center" valign="middle" >Average</td><td align="center" valign="middle" ></td><td align="center" valign="middle" >49.4</td><td align="center" valign="middle" >29.4</td><td align="center" valign="middle" >53.9</td></tr></tbody></table></table-wrap><p>*n = 3.</p><table-wrap id="table5" ><label><xref ref-type="table" rid="table5">Table 5</xref></label><caption><title> The degradation of long-chain c-alkanes by stains harboring no alkB genes in the genus Rhodococcus</title></caption><table><tbody><thead><tr><th align="center" valign="middle"  rowspan="2"  >Strain</th><th align="center" valign="middle"  colspan="3"  >Degradation Ratio (%)*</th></tr></thead><tr><td align="center" valign="middle" >Undecylcyclohexane (UDC)</td><td align="center" valign="middle" >Dodecylcyclohexane (UDC)</td><td align="center" valign="middle" >Tridecylcyclo-hexane (UDC)</td></tr><tr><td align="center" valign="middle" >R. tukisamuensis NBRC100609</td><td align="center" valign="middle" >26.0 &#177; 1.4</td><td align="center" valign="middle" >4.1 &#177; 11.0</td><td align="center" valign="middle" >20.1 &#177; 8.0</td></tr><tr><td align="center" valign="middle" >R. maanshanensis NBRC100610</td><td align="center" valign="middle" >18.3 &#177; 7.4</td><td align="center" valign="middle" >13.4 &#177; 4.2</td><td align="center" valign="middle" >18.6 &#177; 3.4</td></tr><tr><td align="center" valign="middle" >R. pyridinivorans NBRC100608</td><td align="center" valign="middle" >2.0 &#177; 1.4</td><td align="center" valign="middle" >11.2 &#177; 13.1</td><td align="center" valign="middle" >19.6 &#177; 10.2</td></tr><tr><td align="center" valign="middle" >R. kroppenstedtii NBRC103113</td><td align="center" valign="middle" >12.3 &#177; 5.2</td><td align="center" valign="middle" >1.7 &#177; 1.0</td><td align="center" valign="middle" >15.9 &#177; 4.2</td></tr><tr><td align="center" valign="middle" >R. rhodnii NBRC100604</td><td align="center" valign="middle" >34.7 &#177; 11.5</td><td align="center" valign="middle" >34.2 &#177; 3.8</td><td align="center" valign="middle" >34.8 &#177; 3.3</td></tr><tr><td align="center" valign="middle" >R. coprophilus NBRC100603</td><td align="center" valign="middle" >22.2 &#177; 3.3</td><td align="center" valign="middle" >15.4 &#177; 6.7</td><td align="center" valign="middle" >5.6 &#177; 7.1</td></tr><tr><td align="center" valign="middle" >Average</td><td align="center" valign="middle" >19.3</td><td align="center" valign="middle" >13.3</td><td align="center" valign="middle" >19.1</td></tr></tbody></table></table-wrap><p>*n = 3.</p><p>by the alkane hydroxylase gene, while genus Rhodococcus spp. carrying no alkB genes may degrade long-chain c-alkanes by other degradation systems such as those involving the cytochrome gene [<xref ref-type="bibr" rid="scirp.90924-ref41">41</xref>].</p></sec><sec id="s3_5"><title>3.5. Alkane Hydroxylase Gene-Carrying Bacteria and Their Roles in Material Circulation in Nature</title><p>c-alkane-degrading bacteria belonging to the genus Rhodococcus isolated from their natural habitats in our previous study were closely related to R. erythropolis and strains harboring alkB R2 type [<xref ref-type="bibr" rid="scirp.90924-ref33">33</xref>]. Although many c-alkane-degrading bacteria of the genus Rhodococcus are widely distributed, long-chain c-alkanes are limited in the natural environment. These bacteria may utilize hydrocarbons such as the wax of leaves and aromatic compounds, which include linear or cyclic hydrocarbon components. Thus, these bacteria harboring alkane hydroxylase genes might contribute to the material circulation of several kinds of hydrocarbon compounds in nature [<xref ref-type="bibr" rid="scirp.90924-ref42">42</xref>].</p></sec></sec><sec id="s4"><title>4. Conclusion</title><p>To characterize the long-chain cyclic alkane (c-alkane) degradation of bacteria in Rhodococcus, 11 strains were isolated from nature using long-chain c-alkane as a substrate. These strains were identified as R. erythropolis, and all were shown to carry the alkB R2 type. It was also confirmed that the degradation ratios of long-chain c-alkanes by Rhodococcus carrying the alkB R2 type of gene (55.5% of UDC, 31.8% of DDC, and 55.2% of TDC) were higher than in those carrying the non-alkB R2 type of gene (49.4% of UDC, 29.4% of DDC, and 53.9% of TDC) and no alkB gene (19.3% of UDC, 13.3% of DDC, and 19.1% of TDC).</p></sec><sec id="s5"><title>Conflicts of Interest</title><p>The authors declare no conflicts of interest regarding the publication of this paper.</p></sec><sec id="s6"><title>Cite this paper</title><p>Kawagoe, T., Kubota, K., Araki, K.S. and Kubo, M. (2019) Analysis of the Alkane Hydroxylase Gene and Long-Chain Cyclic Alkane Degradation in Rhodococcus. 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