Abstract

The suppressor of Hairy-wing [su(Hw)] protein mediates the mutagenic effect of the gypsy retrotransposon by repressing the function of transcriptional enhancers located distally from the promoter with respect to the position of the su(Hw)-binding region. Mutations in a second gene, modifier of mdg4, also affect the gypsy-induced phenotype. Two major effects of the mod(mdg4)^lul mutation can be distinguished: the interference with insulation by the su(Hw)-binding region and direct inhibition of gene expression that is not dependent on the su(Hw)-binding region position. The mod(mdg4)^lul mutation partially suppresses ct⁶, sc^D1 and Hw¹ mutations, possibly by interfering with the insulation effect of the su(Hw)-binding region. An example of the second effect of mod(mdg4)^lul is a complete inactivation of yellow expression in combination with the y ² allele. Phenotypic analyses of flies with 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 important for direct inhibition of yellow transcription in bristles, while the amino-terminal acidic domain is more essential for insulation.