THE POLYCOMB-GROUP GENE, EXTRA SEX COMBS, ENCODES A NUCLEAR MEMBER OFTHE WD-40 REPEAT FAMILY

We have delimited the extra sex combs (esc) gene to <4 kb that include a single transcript and are able to rescue both the maternal and zygo tic esc phenotypes. Several mutations have been identified within the esc transcript. In agreement with earlier genetic studies, esc is expr essed maternally and its product is most abundant during the early emb ryonic stages. It encodes a protein of the WD-40 repeat family, which localizes predominantly to the nucleus. During germ band extension, it is expressed in a stereotypic pattern of neuroblasts. We propose a mo del in which Esc is recruited by gap proteins both to act as a corepre ssor that competes with the TAF(II)80 coactivator to block transcripti on and also to mediate the transition to permanent repression by Polyc omb-group proteins.