The cAMP-response element-binding protein (CREB)-binding protein and p300 are two highly conserved transcriptional coactivators and histone acetyltransferases that integrate signals from diverse signal transduction pathways in the nucleus and also link chromatin remodeling with transcription. In this report, we have examined the role of p300 in the control of the G 1 phase of the cell cycle in nontransformed immortalized human breast epithelial cells (MCF10A) and fibroblasts (MSU) by using adenovirus vectors expressing p300-specific antisense sequences. Quiescent MCF10A and MSU cells expressing p300-specific antisense sequences synthesized p300 at much reduced levels and exited G1 phase without serum stimulation. These cells also showed an increase in cyclin A and cyclin A-and E-associated kinase activities characteristic of S phase induction. Further analysis of the p300-depleted quiescent MCF10A cells revealed a 5-fold induction of c-MYC and a 2-fold induction of c-JUN. A direct target of c-MYC, CAD, which is requiredfor DNA synthesis, was also found to be up-regulated, indicating that up-regulation of c-MYC functionally contributed to DNA synthesis. Furthermore, S phase induction in p300-depleted cells was reversed when antisense c-MYC was expressed in these cells, indicating that up-regulation of c-MYC may directly contribute to S phase induction. Adenovirus E1A also induced DNA synthesis and increased the levels of c-MYC and c-JUN in serum-starved MCF10A cells in a p300-dependent manner. Our results suggest an important role of p300 in cell cycle regulation at G1 and raise the possibility that p300 may negatively regulate early response genes, including c-MYC and c-JUN, thereby preventing DNA synthesis in quiescent cells.

Although the link between transcription and DNA repair is well established, defects in the core transcriptional complex itself have not been shown to elicit a DNA damage response. Here we show that a cell line with a temperature-sensitive defect in TBP-associated factor 1 (TAF1), a component of the TFIID general transcription complex, exhibits hallmarks of an ATR-mediated DNA damage response. Upon inactivation of TAF1, ATR rapidly localized to subnuclear foci and contributed to the phosphorylation of several downstream targets, including p53 and Chk1, resulting in cell cycle arrest. The increase in p53 expression and the G 1 phase arrest could be blocked by caffeine, an inhibitor of ATR. In addition, dominant negative forms of ATR but not ATM were able to override the arrest in G 1 . These results suggest that a defect in TAF1 can elicit a DNA damage response.The ts13 and tsBN462 cell lines have been used as model systems to study the relationship between transcription and cell cycle control. These lines were originally isolated in screens to identify cells that underwent a cell cycle arrest upon a shift in temperature from 33 to 39°C (28, 45). It was subsequently discovered that both lines contain the same point substitution mutation (G690D) in TBP-associated factor 1 (TAF1) (TAF II 250/CCG1), a key member of the TFIID complex (32,35,36). TAF1 is the largest of several TAFs, forming a scaffold between TBP and other TAFs and contributing to activated transcription (9). The ability of TAF1 to bind to TBP and other TAFs appears to be unaffected in ts13 cells shifted to the restrictive temperature (14, 37). However, TAF1 may lose the ability to bind to TBP at the cyclin D1 promoter in cells shifted to the restrictive temperature (18).TAF1 is associated with at least three enzymatic activities. The N-and C-terminal ends of TAF1 contain a kinase activity that phosphorylates RAP74, a subunit of TFIIF (12). This kinase activity appears to be unaffected by the ts13 mutation (30). The central domain of TAF1 contains a histone acetyltransferase (HAT) activity that can acetylate TFIIE␤ and histones H3 and H4 (21, 26). The ts13 and tsBN462 point mutation in TAF1 is located within the HAT domain, and a mutation of the corresponding residue in human TAF1 resulted in temperature-sensitive elimination of the HAT activity in vitro (14). TAF1 also contains an E1 and E2 ubiquitin activating and conjugating activity that participates in the monoubiquitination of histone H1 (23). It is not known whether this activity of TAF1 is affected by the ts13 and tsBN462 mutation.Since TAF1 contributes to activated transcription, loss-offunction mutations could be expected to have significant effects on the transcription profile. Surprisingly, the ts13 and tsBN462 point mutation affected the transcription of a limited set of genes, including those that encode the cyclin-dependent kinase Cdk1/Cdc2 and cyclins D1, D3, and A (44, 49, 50). The transcription of several other genes, including c-fos and c-myc, was not reduced at the restrictive tempe...

The retinoblastoma tumor suppressor gene product (pRb) is involved in controlling cell cycle progression from G 1 into S. pRb functions, in part, by regulating the activities of several transcription factors, making pRb involved in the transcriptional control of cellular genes. Transient-transfection assays have implicated pRb in the transcription of several genes, including c-fos, the interleukin-6 gene, c-myc, cdc-2, c-neu, and the transforming growth factor ␤2 gene. However, these assays place the promoter in an artificial context and exclude the effects of far 5 upstream regions and chromosomal architecture on gene transcription. In these experiments, we have studied the role of pRb in the control of cell cycle-related genes within a chromosomal context and within the context of the G 1 phase of the cell cycle. We have used adenovirus vectors to overexpress pRb in human osteosarcoma cells and breast cells synchronized in early G 1 . By RNase protection assays, we have assayed the effects of this virus-produced pRb on gene expression in these cells. These results indicate that pRb is involved in the transcriptional downregulation of the E2F-1, E2F-2, dihydrofolate reductase, thymidine kinase, c-myc, proliferating-cell nuclear antigen, p107, and p21/Cip1 genes. However, it has no effect on the transcription of the E2F-3, E2F-4, E2F-5, DP-1, DP-2, or p16/Ink4 genes. The results are consistent with the notion that pRb controls the transcription of genes involved in S-phase promotion. They also suggest that pRb negatively regulates the transcription of two of the transcription factors whose activity it also represses, E2F-1 and E2F-2, and that it plays a role in downregulating the immediate-early gene response to serum stimulation.

At least three domains of simian virus 40 large T antigen (TAg) participate in cellular transformation. The LXCXE motif of TAg binds to all members of the retinoblastoma protein (pRB) family of tumor suppressors. The N-terminal 70 residues of TAg have significant homology to the J domain of Hsp40/DnaJ and cooperate with the LXCXE motif to inactivate the pRB family. A bipartite C-terminal domain of TAg binds to p53 and thereby disrupts the ability of p53 to act as a sequence-specific transcription factor. The contribution of these three domains of TAg to cellular transformation was evaluated in cells that contained inactivating mutations in the pRB and p53 pathways. Cells that stably expressed wild-type or selected mutant forms of TAg were generated in mouse embryo fibroblasts (MEFs) containing homozygous deletions in the RB, INK4a, and ARF loci. It was determined that the J domain, the LXCXE motif, and the p53-binding domain of Simian virus 40 (SV40) large T antigen (TAg) has been used extensively as a model system to study cellular transformation. TAg has the ability to transform a wide variety of normal cells seemingly by affecting the functions of a small number of cellular proteins. To transform wild-type (WT) mouse embryo fibroblasts (MEFs), TAg utilizes at least three domains: the J domain, the LXCXE motif that binds to the retinoblastoma protein (pRB) family of proteins (pRB, p107, and p130), and the p53-binding domain (12,13,17,20,65,77,81). The J domain is a highly conserved element present in all members of the DnaJ/Hsp40 family of molecular chaperones as well as all polyomavirus T antigens (39). DnaJ proteins bind specifically to hsp70 homologues to perform various chaperone activities, including the destruction of specific proteins (reviewed in reference 63). The J domain of TAg binds to hsc70 and participates in the inactivation of pRB family members (27,56,60,68,69). The J domain and the LXCXE motif of TAg cooperate to disrupt the ability of pRB family members to repress E2F-dependent transcription and to decrease the levels of hyperphosphorylated p107 and p130 (27,60,64,65,68,69,77). Thus, the J domain and LXCXE motif of TAg appear to induce transformation and promote cell growth by interfering with the functions of pRB, p107, and p130. The p53-binding domain of TAg binds to the specific DNA-binding domain of p53, thereby directly interfering with the ability of p53 to activate transcription (3,21,51). Therefore, it is believed that TAg can transform cells primarily by interfering with p53 and the pRB family.The original observations that TAg binds to p53 and the pRB family set into motion a large field of research that has led to a more complete understanding of the role of these tumor suppressors in the development of cancer (13,44,46). Not only is pRB itself mutated in a wide variety of cancers, wild-type pRB can be functionally inactivated by expression of an LXCXEcontaining viral oncoprotein. pRB can also be inactivated by hyperphosphorylation as a result of overexpression of cyclin D1 or los...