(CIT): The focus of much of the work in Dr. Prives' laboratory is the functions of p53 and its negative regulator and transcriptional target, Mdm2. Two general areas are being investigated, how p53 functions as a transcription factor and the roles and regulation of Mdm2. P53 is a sequence-specific transcriptional activator of genes involved in cellular outcomes such as cell cycle arrest and apoptosis. We have identified a new co-regulator of select p53 targets, Cas/Cse1L, a protein previously shown to be involved in nuclear transport. Cas/Cse1L binds to the promoters of a subset of p53 targets and levels of Cas/Cse1L are correlated with the extent to which these genes are induced by p53. Atne p53 target promoter, PIG3, Cas/Cse1L has a long range inhibitory effect on tri-methylation of histone H3 lysine 27, a repressive modification, which could explain how Cas/Cse1L and p53 cooperate to regulate some p53 targets and apoptosis. Another project stems from our earlier finding that in some cells p53 is impaired in its ability to induce transcription from some of its target genes when cells are blocked in S phase. We have extended these findings by showing that while p53 can bind to its sites within these promoters and recruit histone acetylases, TFIID and RNA polymerase, transcription elongation by RNA polymerase is significantly reduced when the DNA replication checkpoint is activated. Our data implicate the checkpoint kinases in this block to elongation of p53 targets. Regarding our studies on Mdm2, an extensive yeast two-hybrid screen led to the identification of RPS7, a ribosomal protein that can interact with and regulate Mdm2. Ribosomal stress leads to the appearance of RPS7 in the nucleoplasm and RPS7 inhibits Mdm2 E3 ligase activity in vivo and in vitro. Our data also suggest that the Mdm2 homologue, MdmX, is involved in RPS7 repression of Mdm2. Further, ablation of RPS7 causes both increased turnover of Mdm2 and decreased p53 after treatment of cells wi...