Complementation group C of xeroderma pigmentosum (XP) represents one of the most common forms of this cancer‐prone DNA repair syndrome. The primary defect is located in the subpathway of the nucleotide excision repair system, dealing with the removal of lesions from the non‐transcribing sequences (‘genome‐overall’ repair). Here we report the purification to homogeneity and subsequent cDNA cloning of a repair complex by in vitro complementation of the XP‐C defect in a cell‐free repair system containing UV‐damaged SV40 minichromosomes. The complex has a high affinity for ssDNA and consists of two tightly associated proteins of 125 and 58 kDa. The 125 kDa subunit is an N‐terminally extended version of previously reported XPCC gene product which is thought to represent the human homologue of the Saccharomyces cerevisiae repair gene RAD4. The 58 kDa species turned out to be a human homologue of yeast RAD23. Unexpectedly, a second human counterpart of RAD23 was identified. All RAD23 derivatives share a ubiquitin‐like N‐terminus. The nature of the XP‐C defect implies that the complex exerts a unique function in the genome‐overall repair pathway which is important for prevention of skin cancer.

The transcription factor Klf4 has demonstrated activity in the reprogramming of somatic cells to a pluripotent state, but the molecular mechanism of this process remains unknown. It is, therefore, of great interest to understand the functional role of Klf4 and related genes in ESC regulation. Here, we show that homozygous disruption of Klf5 results in the failure of ESC derivation from ICM cells and early embryonic lethality due to an implantation defect. Klf5 KO ESCs show increased expression of several differentiation marker genes and frequent, spontaneous differentiation. Conversely, overexpression of Klf5 in ESCs suppressed the expression of differentiation marker genes and maintained pluripotency in the absence of LIF. Our results also suggest that Klf5 regulates ESC proliferation by promoting phosphorylation of Akt1 via induction of Tcl1. These results, therefore, provide new insights into the functional and mechanistic role of Klf5 in regulation of pluripotency.

Background: Smad proteins are novel transcriptional regulators mediating the signalling of the transforming growth factor-␤ (TGF-␤) superfamily. Coactivators such as p300/CBP promote transactivation by various transcription factors through a direct interaction with them. Adenoviral oncoprotein E1A, which binds p300, was shown to inhibit the signalling of TGF-␤. These findings raise the possibility that p300 may be involved in TGF-␤ signalling.

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Smad proteins are intracellular signalling mediators of transforming growth factor-β β β β (TGF-β β β β ) superfamily. In the nucleus, activated Smad complexes regulate transcriptional responses of the target genes in cooperation with transcriptional coactivators and corepressors. To identify new components of transcriptional complexes containing Smad proteins, we purified DNA-binding proteins from human breast cancer MCF-7 cell nuclear extract using a Smad-binding DNA element as bait, and identified a coactivator GCN5 as a direct partner of activated Smad complexes. GCN5 is structurally similar to PCAF, which was previously identified as a coactivator for receptorregulated Smads (R-Smads) for TGF-β β β β signalling pathways. GCN5 functions like PCAF, in that it binds to TGF-β β β β -specific R-Smads, and enhances transcriptional activity induced by TGF-β β β β . In addition, GCN5, but not PCAF, interacts with R-Smads for bone morphogenetic protein (BMP) signalling pathways, and enhances BMP-induced transcriptional activity, suggesting that GCN5 and PCAF have distinct physiological functions in vivo . Moreover, silencing of the GCN5 gene by RNA interference results in repression of transcriptional activities induced by TGF-β β β β . In conclusion we identified GCN5 as a Smad-binding transcriptional coactivator which positively regulates both TGF-β β β β and BMP signalling pathways.