Analysis of a Polycomb Group Protein Defines Regions That Link Repressive Activity on Nucleosomal Templates to In Vivo Function

Abstract

Polycomb group (PcG) genes propagate patterns of transcriptional repression throughout development. The products of several such genes are part of Polycomb repressive complex 1 (PRC1), which inhibits chromatin remodeling and transcription in vitro. Genetic and biochemical studies suggest the product of the Posterior sex combs (Psc) gene plays a central role in both PcG-mediated gene repression in vivo and PRC1 activity in vitro. To dissect the relationship between the in vivo and in vitro activities of Psc, we identified the lesions associated with 11 genetically characterized Psc mutations and asked how the corresponding mutant proteins affect Psc activity on nucleosomal templates in vitro. Analysis of both single-mutant Psc proteins and recombinant complexes containing mutant protein revealed that Psc encodes at least two functions, complex formation and the inhibition of remodeling and transcription, which require different regions of the protein. There is an excellent correlation between the in vivo phenotypes of mutant Psc alleles and the structure and in vitro activities of the corresponding proteins, suggesting that the in vitro activities of PRC1 reflect essential functions of Psc in vivo.

ACKNOWLEDGMENTS

We thank P. Adler for anti-Psc antibodies and J. Bateman, A. Lee, J. Lokere, A. Moran, S. Ou, B. Williams, and the Dana Farber/Harvard Cancer Center sequencing facility for discussion and technical assistance.

This work was supported by the NIH (R.E.K., C.-T.W., and R.B.E.) and the Charles King Trust (N.J.F.).