TITLE:
RNA species whose transcription is totally silent in pre-MBT stage are not mRNA but rRNA and possible involvement of weak bases (ammonium salts and/or amines) in the transcriptional silence of rRNA genes during the pre-MBT stage in Xenopus early embryos
AUTHORS:
Koichiro Shiokawa
KEYWORDS:
Pre-MBT Transcription; Absence of rRNA Synthesis; Initiation of rRNA Synthesis; Nucleolus Formation; Weak Bases; Amines; Ammonium Ion; Xenopus Embryogenesis
JOURNAL NAME:
Advances in Bioscience and Biotechnology,
Vol.4 No.10B,
October
8,
2013
ABSTRACT: In Xenopus laevis embryogenesis, fertilized eggs undergo 12 cycles of synchronous divisions and
reach the stage called midblastula transition (MBT). It has long been believed
that during the first 12 cycles of cleavage (pre-MBT stage), transcriptional
activity of the zygotic nuclei is totally absent. However, heterogeneous
mRNA-like RNA is synthesized in pre-MBT stage embryos, and exogenously-injected
bacterial CAT genes with SV40 promoter are
expressed from the cleavage stage. Nevertheless, the synthesis of rRNA
as detected by rRNA-specific2’-O-methylation
does not take place in pre-MBT embryos and starts only from the latter half of
the MBT stage, corroborating the fact that formation of definitive nucleoli as
well as the transcription of microinjected rRNA genes starts only at and after
MBT stage. Thus, while mRNA-like RNA synthesis occurs from pre-MBT stage,
synthesis of rRNA is controlled in the way that transcription of rRNA genes is
totally silent during pre-MBT stage and is initiated only at the latter half of
MBT stage. Once initiated, the rate of the synthesis of rRNA is constant throughout
later stages on a per-cell basis. We searched substances which are responsible
for the transcriptional silence of rRNA genes during the pre-MBT stage. Weak
bases such as ammonium ion and amines selectively inhibited rRNA synthesis at
the transcriptional level in post-MBT stage embryo cells. Since we found that
the level of ammonia extracted from embryos is much higher in pre-MBT embryos
than in post-MBT embryos, we suggest that weak bases like ammonium ion could be
responsible for the transcriptional silence of rRNA genes by slightly increasing
intracellular pH during the pre-MBT.