POLYCOMB AND BMI-1 HOMOLOGS ARE EXPRESSED IN OVERLAPPING PATTERNS IN XENOPUS EMBRYOS AND ARE ABLE TO INTERACT WITH EACH OTHER

The Polycomb group genes in Drosophila are involved in the stable and inheritable repression of gene expression. The Polycomb group proteins probably operate as multimeric complexes that bind to chromatin. To i nvestigate molecular mechanisms of stable repression of gene activity in vertebrates we have begun to study Xenopus homologs of Polycomb gro up genes. We identified the Xenopus homologs of the Drosophila Polycom b gene and the bmi-1 gene. bmi-1 is a proto-oncogene which has sequenc e homology with the Polycomb group gene Posterior Sex Combs. We show t hat the XPolycomb and Xbmi-1 genes are expressed in overlapping patter ns in the central nervous system of Xenopus embryos. However, XPolycom b is also expressed in the somites, whereas Xbmi-1 is not. We further demonstrate that the XPolycomb and Xbmi-1 proteins are able to interac t with each other via conserved sequence motifs. These data suggest th at also vertebrate Polycomb group proteins form multimeric complexes.