Mutations in Drosophila heat shock cognate 4 are enhancers of Polycomb

The homeotic genes controlling segment identity in Drosophila are repressed by the Polycomb group of genes (PcG) and are activated by genes of the tri thorax group (trxG), An F-1 screen for dominant enhancers of Polycomb yield ed a point mutation in the heat shock cognate gene, hsc4, along with mutati ons corresponding to several known PcG loci. The new mutation is a more pot ent enhancer of Polycomb phenotypes than an apparent null allele of hsc4 is , although even the null allele occasionally displays homeotic phenotypes a ssociated with the PcG, Previous biochemical results had suggested that HSC 4 might interact with BRAHMA, a trxG member. Further analyses now show that there is no physical or genetic interaction between HSC4 and the Brahma co mplex. HSC4 might be needed for the proper folding of a component of the Po lycomb repression complex, or it may be a functional member of that complex .