We analyzed the accumulation of newly-synthesized heterogeneous mRNA-like RNA, 4 S RNA, 5 S RNA, snRNAs and rRNA before and after the midblastula transition (MBT) in Xenopus laevis embryogenesis. Based on the kinetics of the labeling, we concluded that the pattern of RNA synthesis in Xenopus embryogenesis changes following at least three characteristically different phases. The first phase is the pre-MBT stage, which is characterized by the synthesis of heterogeneous mRNA-like RNA, accompanied by the synthesis of small amounts of 4 S RNA, 5 S RNA and snRNAs. The second phase is the MBT stage which is characterized by a large activation (about 50-fold increase on a per cell basis) of 4 S RNA synthesis. The third phase is the post-MBT stage which is characterized by the commencement and increase in rRNA synthesis. We assume that RNA polymerases II, III and I are activated in this order in early Xenopus embryogenesis.
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