THE EXTRA SEX COMBS PRODUCT, CONTAINS WD40 REPEATS AND ITS TIME OF ACTION IMPLIES A ROLE DISTINCT FROM OTHER POLYCOMB GROUP PRODUCTS

The extra sex combs (esc) gene product is a transcriptional repressor of homeotic genes. Although it is classified in the Polycomb group (Pc G) on the basis of phenotypic criteria, it is distinct from most other PcG repressors in its time of action during development. We describe the temporal profile of esc mRNA expression during embryogenesis and t he stage-specific rescue of esc mutants with a heat shock-inducible es e cDNA transformation construct. Both experiments support the idea tha t ese product plays an early, transient role in repression of homeotic genes. We also present the sequence of a full-length ese cDNA. The pr edicted esc protein is composed primarily of multiple copies of a repe at motif, termed the WD40 repeat, which are likely used in protein-pro tein contact. We provide evidence that individual copies of the esc WD 40 repeats are needed for function in vivo. We suggest that esc protei n is an adaptor that binds to multiple protein partners and assists in the assembly or targeting of other PcG proteins.