GENETIC MODULATION OF RNA METABOLISM IN DROSOPHILA. III. REQUIREMENT FOR AN rDNA-DEFICIENT X CHROMOSOME IN Y^bbSuVar–5-MEDIATED INCREASES IN RNA SYNTHESIS

Linda D. Strausbaugh ¹ and Barry I. Kiefer ¹

¹ Department of Biology, Wesleyan University, Middletown, Connecticut 06457

Males of the genotype car bb/Y^bbSuVar–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/Y^bb- (Clark, Strausbaugh and Kiefer 1977; Clark and Kiefer 1977). Experiments were designed to determine if the modulation of RNA metabolism by Y^bbSuVar–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/Y^bbSuVar–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 Y^bbSuVar–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/Y^bbSuVar–5 males and Sam⁺iso/Y^bb- males have equivalent amounts of rDNA.——These results are consistent with the suggestion that the Y^bb- and Y^bbSuVar–5 chromosomes have similar amounts of rDNA. The nature of compensatory strategies in rDNA-deficient genotypes is discussed.