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

Abstract
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.