The retinoblastoma gene (Rb) is a master regulator of the cell cycle, capable of silencing target genes that mediate cell entry into the S phase. The Rb protein acts by binding to a family of transcription regulators termed the E2Fs. Upon binding of Rb to E2F, the complex attaches to regulatory sequences of E2F/Rb target genes and suppresses their expression. This suppression is effected, at least in part, by recruitment of histone deacetylases, which leads to the formation of a condensed chromatin structure at the target gene locus.

In the August 2, 2001 issue of Nature, Nielsen and colleagues show that Rb-mediated repression might also take place through methylation of histones at E2F/Rb-regulated gene promoters. It has recently been demonstrated that the mammalian histone methylase SUV39H1 (specific for lysine 9 of histone H3) and the associated methyl-lysine binding protein HP1 can repress gene expression. Nielsen and colleagues now demonstrate that the Rb protein binds to SUV39H1/HP1 and in doing so, recruits the complex to the E2F/Rb-regulated gene promoters to mediate repression.

Experimental Approach

In order to show that the Rb protein associates with SUV39H1 and HP1 in cells, the authors used immunoprecipitation experiments in conjunction with Western blots. They then demonstrated that Rb could recruit the SUV39H1/HP1 complex to a specific E2F/Rb-regulated promoter by immunoprecipitating chromatin with antibodies to HP1 or lys 9-methylated H3 histone followed by PCR. Finally, the authors established that the Rb/SUV39H1/HP1 complex could repress E2F/Rb-specific target genes by transient co-transfection of expression and reporter gene constructs.

The Model

These experiments led the authors to construct a more detailed model for Rb-mediated gene suppression. In this model, the Rb protein first recruits the histone deacetylase complex to E2F/Rb-regulated gene promoters. Once the histones associated with the promoter are deacetylated, the SUV39H1/HP1 complex is recruited to the promoter site. The SUV39H1 protein then methylates the promoter-associated H3 histone at lysine 9. This order of recruiting is logical because SUV39H1 methylation cannot take place on an acetylated histone. It is the combination of histone deacetylation and methylation that leads to gene inactivation. This repression may to some extent be mediated by chromatin condensation.

As alluded to by the authors, the HP1 protein binds to a number of transcriptional repressors. Therefore repression through histone methylation may prove to be a commonly used mode of gene regulation.

Reference

Nielsen, S.J., Schneider, R., Bauer, U-M., Bannister, A.J., Morrison, A., O'Carroll, D., Firestein, R., Cleary, M., Jenuwein, T., Herrera, R.E., and Kouzarides, T. (2001) Rb targets histone H3 methylation and HP1 to promoters. Nature 412: 561-565.