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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.

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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 &amp;lt;i&amp;gt;Xenopus&amp;lt;/i&amp;gt; early embryos