Demonstration of rRNA Synthesis in Pre-Gastrular Embryos of Xenopus Laevis

Isolated cells from Xenopus laevis embryos at various developmental stages were incubated with (methyl-³H) methionine to label newly synthesized RNAs. Methylation of RNAs was studied by analyzing nuclease-digests of high-molecular-weight RNAs by DEAE-Sephadex column chromatography. Labeled rRNA, mRNA and 4s RNA were distinguished by their characteristic patterns of 2'-0-methylation and base-methylation. It was concluded that rRNA synthesis starts during the mid- to late-blastula stage. This is about 4 hr, or at least 3 cell cycles earlier than the initiation of gastrulation, which was previously considered to be the stage when rRNA synthesis begins.