Two-hybrid screen for Corto partners. J. Salvaing, E. Mouchel-Vielh, F. Peronnet. CNRS-UPMC, Paris, FRANCE.

   Polycomb (Pc-G) and trithorax (trx-G) group genes are involved in regulation of gene expression during development by modifying chromatin structure. They have been essentially studied for their role in homeotic genes regulation. Pc-G and Trx-G proteins act as multimeric complexes at chromatin level. Many arguments suggest that Pc-G complexes, which are repressors, compact chromatin, while Trx-G complexes, which are activators, either prevent the spreading of chromatin compaction or participate in chromatin remodelling. The corto gene encodes a chromodomain protein which interacts with the GAGA factor and co-immunoprecipitates with PC and ESC. As other Pc-G and Trx-G proteins, Corto is found at many sites on third instar larvae polytene chromosomes, which suggests that it controls many targets. Among these targets is the homeotic gene Abd-B . We performed a 2-hybrid screen to detect Corto new partners. We will present these partners. One of those is Cyclin G, a cyclin which role in Drosophila remains mainly unknown. Preliminary genetic analysis suggests that the Corto-Cyclin G interaction is involved in the epithelium formation of the abdominal segments A5 and A6, which could be linked to the regulation of Abd-B by Corto.