Timing of the initiation of rRNA gene expression and nucleolar formation in cleavage embryos arrested by cytochalasin B and podophyllotoxin and in cytoplasm-extracted embryos of Xenopus laevis.

The timing mechanism of rRNA gene expression was studied in Xenopus laevis embryos whose cleavage was arrested by treatment with cytochalasin B or podophyllotoxin, and in fertilized eggs from which about 40% of the cytoplasm was extracted by micromanipulation. rRNA synthesis was activated in the cleavage-arrested embryos and also in cytoplasm-extracted embryos at the time when the control embryos commenced synthesizing rRNA at the late blastula stage. Formation of the nucleolar structure occurred at the normal time in the nuclei of cleavage embryos arrested by podophyllotoxin. Therefore, neither twelve continuous cell divisions nor the nuclear division which normally precedes the MBT (mid-blastula transition) was the factor determining the timing of rDNA expression. Moreover, lowering of the cytoplasmic:nuclear ratio to a certain threshold level was not a prerequisite for rDNA activation. Some regulatory mechanism dependent on the time elapsed from fertilization is likely to be the important factor permitting rDNA expression.