Affecting the Phenotype of gylbS3FInduced Mutations

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, modzjier of mdg4, also s e c t the gypsyinduced phenotype. Two major effects of the mod(mdg4)'"' 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)'"' mutation partially suppresses ct', s?' and Hw' mutations, possibly by interfering with the insulation effect of the su(Hw)-binding region. An example of the second effect of mod(mdg4)'"' is a complete inactivation of yellow expression in combination with the y2 allele. Phenotypic analyses of flies with combinations of mod(mdg4)'"' 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