The Posterior Sex Comb (Psc) gene of Drosophila is a pleiotropic developmental regulatory gene. As a member of the Polycomb group of genes it is essential for the maintenance of segment specific embryonic determination. We have cloned and sequenced this gene and found that it shares a 200 amino acid domain with the neighboring Suppressor Two of zeste (Su(z)2) gene and the recently described murine bmi-1 oncogene. Using immunostaining of polytene chromosomes we have found that the Psc protein is a locus specific chromosomal protein that is found at the location of a number of expected target genes. We will use this chromosome staining assay and germ line transformation to map the target DNA sequences that are responsible for the chromosomal binding site and we will determine if the Psc protein can directly bind to these sequences in vitro. We will attempt to determine the function of the protein domain shared with Su(z)2 and bmi-1 using in vitro mutagenesis and chimeric genes combined with assays for mutant rescue, chromosomal binding, and if possible DNA binding in vitro. We will also continue our analysis of the developmental function of Psc. We will use antibody staining and lacz expression from enhancer traps in embryos that lack both maternal and zygotic Psc function to determine if it is the initiation or maintenance of gene expression patterns that is effected in several of the developmental pathways that are perturbed by a lack of Psc function.***//
