THE SU(HW) PROTEIN-BOUND TO GYPSY SEQUENCES IN ONE CHROMOSOME CAN REPRESS ENHANCER PROMOTER INTERACTIONS IN THE PAIRED GENE LOCATED IN THE OTHER HOMOLOG

The suppressor of Hairy-wing [su(Hw)] protein exerts a polar effect on gene expression by repressing the function of transcriptional enhance rs located distally from the promoter with respect to the location of su(Hw) binding sequences. The directionality of this effect suggests t hat the su(Hw) protein specifically interferes with the basic mechanis m of enhancer action. Moreover, mutations in modifier of mdg4 [mod(mdg 4)] result in the repression of expression of a gene when the su(Hw) p rotein is bound to sequences in the copy of this gene located in the h omologous chromosome. This effect is dependent on the presence of the su(Hw) binding region from the gypsy retrotransposon in at least one o f the chromosomes and is enhanced by the presence of additional gypsy sequences in the other homology. This phenomenon is inhibited by chrom osomal rearrangements that disrupt pairing, suggesting that close appo sition between the two copies of the affected gene is important for tr ans repression of transcription. These results indicate that, in the a bsence of mod(mdg4) product, the su(Hw) protein present in one chromos ome can act in trans and inactivate enhancers Located in the other hom olog.