Genetic Modulation of RNA Metabolism in Drosophila. III. Requirement for an Rdna-Deficient X Chromosome in YbbSuVar–5-Mediated Increases in RNA Synthesis

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Males of the genotype car bb/YbbSuVar–5 have an approximately two-fold increase in the rate of accumulation of 4S, 5S, 18S + 28S and poly-A+ RNA molecules when compared to males of the genotype car bb/Ybb- (Clark, Strausbaugh and Kiefer 1977; Clark and Kiefer 1977). Experiments were designed to determine if the modulation of RNA metabolism by YbbSuVar–5 requires an rDNA deficiency on the X chromosome. Synthetic rates for 4S, 5S, and 18S + 28S RNA's were measured in the genotypes Sam+iso/Ybb- and Sam+iso/YbbSuVar–5 by injecting adult males with 3H-uridine, allowing them to incorporate label for two hours, fractionating extracted RNA by polyacrylamide gel electrophoresis and determining the specific activities of RNA's (dpm/µgRNA). The results of these determinations showed that the synthetic rates for 4S, 5S, and 18S + 28S RNA's are the same in these two genotypes, demonstrating that the YbbSuVar–5 chromosome does not increase transcription when paired with an X chromosome that is wild type for rDNA. Saturation hybridization was used to measure the rDNA content in several genotypes. These results showed that Sam+iso/YbbSuVar–5 males and Sam+iso/Ybb- males have equivalent amounts of rDNA.——These results are consistent with the suggestion that the Ybb- and YbbSuVar–5 chromosomes have similar amounts of rDNA. The nature of compensatory strategies in rDNA-deficient genotypes is discussed.

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