Abstract

By the technique of artificial RNA-DNA hybridization the fraction of Xenopus laevis wild type DNA complementary to homologous 28-S and 18-S ribosomal RNA is found to be 0.07 % and 0.04 % respectively. The ribosomal RNA-DNA complex, at saturation, possesses a buoyant density of approximately 1.77 g·cm−3 in CsCl. Complete separation of the hybrid from the non-complexed denatured DNA is achieved on a CsCl density gradient. The ribosomal RNA-DNA hybrid is resistant to ribonuclease (EC 2.7.7.16), is of high G+C-content and dissociates at high temperature only. By fractionating Xenopus DNA prior to annealing it may be shown that the 28-S ribosomal RNA complexes only with a high density fraction of the Xenopus DNA. 28-S RNA does not anneal to bacterial DNA even of similar G+C composition. DNA from homozygous, anucleolate mutants of Xenopus anneals very poorly to ribosomal RNA, while heterozygous DNA complexes at levels intermediate between those of homozygous mutant and wild type DNA. The potential number of nucleoli, the number of secondary constrictions (presumably nucleolar organizers) and the number of ribosomal cistrons, all show a linear reduction in proportion to the dosage of mutation. This finding strongly suggests that the anucleolate Xenopus is a deletion mutant, and supports the hypothesis that ribosomal cistrons are located at the nucleolar organizer region.