Histone deacetylase-dependent transcriptional repression by pRB in yeast occurs independently of interaction through the LXCXE binding cleft

We have developed a yeast model system to address transcriptional repressio n by the retinoblastoma protein (pRB). When fused to the DNA-binding domain of Gal4p (DB-pRB), pRB can repress transcription of reporter genes contain ing Gal4p binding sites: the histone deacetylase activity encoded by yeast RPD3 is required for DB-pRB repression. Mutation of the LXCXE binding cleft in pRB, a region reported to be required for histone deacetylase recruitme nt, does not interfere with pRB-mediated repression. From these findings ba sed on yeast experiments, we surmise that the small pocket region of pRB mu st contain an additional domain that confers histone deacetylase-dependent transcriptional repression. This hypothesis was verified by experiments exa mining pRB-dependent histone deacetylase association in mammalian cells. In addition to RPD3, repression by pRB in yeast requires MS/1, an ortholog of RbAp48, hut not SIN3 or SAP30. By comparing the genetic requirements of DB -pRB repression in yeast to those of other DB-repressor fusions, we can sug gest a mechanism by which pRB recruits histone deacetylase activity.