The Additional sex combs gene of Drosophila is required for activation andrepression of homeotic loci, and interacts specifically with Polycomb and super sex combs

The protein products of Polycomb group (PcG) and trithorax group (trxG) gen es are required for the maintenance of the transcriptionally repressed and active states, respectively, of the homeotic genes. Mutations in PcG genes produce gain-of-function (posterior) homeotic transformations, while mutati ons in trxG genes produce loss-of-function (anterior) homeotic transformati ons. Double mutant combinations between a PcG gene and a trxG gene suppress the homeotic transformations seen with either mutation alone, suggesting t hat PcG and trxG genes act antagonistically. The PcG gene Additional sex co mbs (Asx) is interesting because one mutant allele, Asx(P1), causes both an terior and posterior homeotic transformations. Asx(P1) and other Asx mutati ons were crossed to mutations in the PcG gene Polycomb (Pc) and the trxG ge ne trithorax (trx). Asx alleles enhance both PcG and trxG homeotic transfor mations, showing that Asx is required for both the activation and the repre ssion of homeotic loci. Asx also shows strong allele-specific interactions with the PcG genes Pc and super sex combs (sxc). Together, these data indic ate that there are functional interactions between Asx, Pc and sxc in vivo. ASX may interact with a PcG complex containing PC and SXC and mediate acti vation Versus repression at target loci, perhaps by interacting directly wi th the TRX protein.