Characterization of functional domains of the su(Hw) protein that mediate the silencing effect of mod(mdg4) mutations.

The suppressor of Hairy-wing [su(Hw)] protein represses enhancer function in a unidirectional fashion: enhancers segregated from the promoter by the su(Hw) binding region are rendered inactive. whereas those in the same domain are unaffected. In the case of the gypsy-induced y2 allele, the repressive effect of su(Hw) is rendered bidirectional in mod(mdg4) mutant flies, and all enhancers of the affected gene become inactive. This silencing of enhancer elements might be due to exposure of specific domains of su(Hw) when the mod(mdg4) protein is absent. Two of three regions of su(Hw) that are located adjacent to the leucine zipper motif and are conserved across Drosophila species are necessary for both the unidirectional and bidirectional repression of transcription by su(Hw). In contrast, two acidic domains that are dispensable for the unidirectional repression of enhancer elements are critical for the bidirectional silencing of enhancer activity observed in mutants lacking functional mod(mdg4) protein.