ABSENCE OF RIBOSOMAL RNA SYNTHESIS IN THE ANUCLEOLATE MUTANT OF XENOPUS LAEVIS

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Abstract
A mutation in Xenopus laevis that prevents the formation of a normal nucleolus at the same time prevents the synthesis of 28S and 18S ribosomal RNA as well as high molecular weight ribosomal RNA precursor molecules. DNA, 4S RNA, and rapidly labeled heterogeneous RNA are synthesized by the anucleolate mutant. Anucleolate mutants survive until the swimming tadpole stage and show normal differentiation of all the main cell types despite their inability to synthesize new ribosomal RNA. Homozygous mutants (0-nu) and control embryos conserve the ribosomes made during oogenesis and associate rapidly synthesized RNA with these old ribosomes. The 28S and 18S ribosomal RNA''s differ in base composition and are probably products of different genes; yet their synthesis is coordinate. In the heterozygous (1-nu) embryos, the wild-type genes regulate to produce twice as much 28S and 18S ribosomal RNA as do the same genes when present in homozygous wild-type individuals. Since the activity of the entire complement of genes determining ribosomal RNA structure can be curtailed by a single mutation, it is suggested that these genes are under common control and located at the "nucleolar organizer" site of a single chromosome.