Inhibitor of Ribosomal RNA Synthesis in Xenopus laevis Embryos. VII. Inhibition of 40S Pre-rRNA Synthesis in Xenopus Neurula Cells

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A fraction containing acid-soluble materials (PCA-extract) from Xenopus early blastulae preferentially inhibits the incorporation of [³H]uridine into 18S and 28S rRNA in Xenopus neurula cells. Pulse-laneling experiments revealed that whereas tubercidin, a known inhibitor of rRNA processing, produced a marked accumulation of the label in 40S pre-rRNA, Xenopus inhibitor suppressed labeling of the pre-rRNA. When tubercidin was added to cells whose activity for rRNA synthesis had been lowered by the Xenopus inhibitor, there was still an accumulation of label in the pre-rRNA. These results indicate that Xenopus inhibitor suppresses the synthesis of rRNA at transcription rather than at processing.