The Polycomb protein shares a homologous domain with a heterochromatin-associated protein of Drosophila.

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The Polycomb (Pc) gene of Drosophila melanogaster is a member of a large class of genes (Pc group) required for the segment-specific repression of homeotic selector genes. Mutations in Pc-group genes show strong posterior transformations in homozygous embryos resulting from an ectopic expression of homeotic genes in segments where they are not supposed to be active. Genetic evidence suggests that Pc is part of a cellular memory mechanism responsible for the transmission of the homeotic expression pattern through developmental time. We have determined the nucleotide sequence for the genomic DNA of the Pc gene and for cDNAs corresponding to the 2.5-kilobase Pc mRNA. The deduced sequence of the Pc protein exhibits a homology to the heterochromatin-associated protein HP1 encoded by the suppressor of position effect variegation gene Su(var) 205. The homology is confined to a 37-amino acid domain in the N-terminal part of the two proteins. Our findings extend to the molecule level the genetically identified parallels between the Pc-group genes and the modifiers of position effect variegation. This suggests that Pc could use analogous mechanisms at the level of the higher order chromatin structure for the stable transmission of a determined state, as has been proposed for the clonal propagation of heterochromatin domains.

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