Studies on the mechanism of heterochromatic position effect at the rosy locus of Drosophila melanogaster.

Experiments are described that extend the characterization of position effect variants of the rosy locus and test possible mechanisms of heterochromatic position effect. Rosy position effect variants exhibit a variegated phenotype with respect to xanthine dehydrogenase activity in malpighian tubules. The breakpoints of the position effect mutations are located on the DNA map of the rosy region outside of the rosy locus DNA; ryps11136 is located in the DNA of the l(3)S12 gene immediately proximal to rosy, whereas ryps1149 is located some 15 kb distal to rosy in the pic locus. Southern blot experiments are described that test and reject the notion that heterochromatic position effect results from underreplication of the position-affected gene. Rather, the results of Northern blots serve to direct attention to position effect as a defect in transcription. Histone region deletion heterozygosity and butyrate-feeding experiments failed to exhibit specific suppression of position effect at the rosy locus.