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 funct ion in a unidirectional fashion: enhancers segregated from the promote r by the su(Hw) binding region are rendered inactive, whereas chose in the same domain are unaffected. In the case of the gypsy-induced y(2) allele, the repressive effect of su(Hw) is rendered bidirectional in mod(mdg4)mutant flies, and all enhancers of the affected gene become i nactive. This silencing of enhancer elements might be due to exposure of specific domains of su(Hw) when the mod(mdg4) protein is absent. Tw o 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 transcrip tion by su(Hw). In contrast, two acidic domains that are dispensable f or the unidirectional repression of enhancer elements are critical for the bidirectional silencing of enhancer activity observed in mutants lacking functional mod(mdg4) protein.