THE POLYCOMB-GROUP GENE, EXTRA SEX COMBS, ENCODES A NUCLEAR MEMBER OF THE 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 zygotic esc phenotypes. Several mutations have been identified within the esc transcript. In agreement with earlier genetic studies, esc is expressed maternally and its product is most abundant during the early embryonic 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 model in which Esc is recruited by gap proteins both to act as a corepressor that competes with the TAF(II)80 coactivator to block transcription and also to mediate the transition to permanent repression by Polycomb-group proteins.