Genetic modulation of RNA metabolism in Drosophila. III. Requirement for an rDNA-deficient X chromosome in YbbSuVar-3-mediated increases in RNA synthesis.

Males of the genotype car bb/YbbSuVar-5 have an approximately two-fold increase in the rate of accumulation of 4S, 5S, 18S plus 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 plus 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/microgram RNA). The results of these determination showed that the synthetic rates for 4S, 5S, and 18S plus 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.