CIS-EFFECTS OF HETEROCHROMATIN ON HETEROCHROMATIC AND EUCHROMATIC GENE ACTIVITY IN DROSOPHILA-MELANOGASTER

Chromosomal rearrangements that juxtapose heterochromatin and euchroma tin can result in mosaic inactivation of heterochromatic and euchromat ic genes. This phenomenon, position effect variegation (PEV), suggests that heterochromatic and euchromatic genes differ in their regulatory requirements. This report describes a novel method for mapping region s required for heterochromatic genes, and those that induce PEV of a e uchromatic gene. P transposase mutagenesis was used to generate deriva tives of a translocation that variegated for the light(+) (lt(+)) gene and carried the euchromatic white(+) (w(+)) gene on a transposon near the heterochromatin-euchromatin junction. Cytogenetic and genetic ana lyses of the derivatives showed that P mutagenesis resulted in deletio ns of several megabases of heterochromatin. Genetic and molecular stud ies showed that the derivatives shared a euchromatic breakpoint but di ffered in their heterochromatic breakpoint and their effects on seven heterochromatic genes and the w(+) gene. Heterochromatic genes differe d in their response to deletions. The lt(+) gene was sensitive to the amount of heterochromatin at the breakpoint but the heterochromatic 40 Fa gene was not. The severity of variegated w(+) phenotype did not dep end on the amount of heterochromatin in cis, but varied with local het erochromatic environment. These data are relevant for considering mech anisms of PEV of both heterochromatic and euchromatic genes.