INTERACTION BETWEEN MUTATIONS IN THE SUPPRESSOR OF HAIRY WING AND MODIFIER OF MDG4 GENES OF DROSOPHILA-MELANOGASTER AFFECTING THE PHENOTYPEOF GYPSY-INDUCED MUTATIONS

The suppressor of Hairy-wing [su(Hw)] protein mediates the mutagenic e ffect of the gypsy retrotransposon by repressing the function of trans criptional enhancers located distally from the promoter with respect t o the position of the su(Hw)-binding region. Mutations in a second gen e, modifier of mdg4, also affect the gypsy-induced phenotype. Two majo r effects of the mod(mdg4)(lul) mutation can be distinguished: the int erference with insulation by the su(Hw)-binding region and direct inhi bition of gene expression that is not dependent on the su(Hw)-binding region position. The mod(mdg4)(lul) mutation partially suppresses ct(6 ) sc(D1) and Hw(1) mutations, possibly by interfering with the insulat ion effect of the su(Hw)-binding region. An example of the second effe ct of mod(mdg4)(lul) is a complete inactivation of yellow expression i n combination with the gamma(2) allele. Phenotypic analyses of flies w ith combinations of mod(mdg4)(lul) and different su(Hw) mutations, or with constructions carrying deletions of the acidic domains of the su( Hw) protein, suggest that the carboxy-terminal acidic domain is import ant for direct inhibition of yellow transcription in bristles, while t he amino-terminal acidic domain is more essential for insulation.