THE ADDITIONAL SEX COMBS GENE OF DROSOPHILA ENCODES A CHROMATIN PROTEIN THAT BINDS TO SHARED AND UNIQUE POLYCOMB GROUP SITES ON POLYTENE CHROMOSOMES

The Additional sex combs (Asx) gene of Drosophila is a member of the P olycomb group of genes, which are required for maintenance of stable r epression of homeotic and other loci. Asx is unusual among the Polycom b group because: (1) one Asx allele exhibits both anterior and posteri or transformations; (2) Asx mutations enhance anterior transformations of trx mutations; (3) Asx mutations exhibit segmentation phenotypes i n addition to homeotic phenotypes; (4) Asx is an Enhancer of position- effect variegation and (5) Asx displays tissue-specific derepression o f target genes. Asx was cloned by transposon tagging and encodes a pro tein of 1668 amino acids containing an unusual cysteine cluster at the carboxy terminus. The protein is ubiquitously expressed during develo pment. We show that Asx is required in the central nervous system to r egulate Ultrabithorax. ASX binds to multiple sites on polytene chromos omes, 70% of which overlap those of Polycomb, polyhomeotic and Polycom blike, and 30% of which are unique. The differences in target site rec ognition may account for some of the differences in Asx phenotypes rel ative to other members of the Polycomb group.