The p53 tumor suppressor that plays a central role in the cellular response to genotoxic stress was suggested to be associated with the DNA repair machinery which mostly involves nucleotide excision repair (NER). In the present study we show for the first time that p53 is also directly involved in base excision repair (BER). These experiments were performed with p53 temperature-sensitive (ts) mutants that were previously studied in in vivo experimental models. We report here that p53 ts mutants can also acquire wild-type activity under in vitro conditions. Using ts mutants of murine and human origin, it was observed that cell extracts overexpressing p53 exhibited an augmented BER activity measured in an in vitro assay. Depletion of p53 from the nuclear extracts abolished this enhanced activity. Together, this suggests that p53 is involved in more than one DNA repair pathway.z 1999 Federation of European Biochemical Societies.

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Objective:The multipotential nature of stem or progenitor cells apparently makes them the ideal choice for any cell therapy, but this as yet remains to be proven. This study (30 subjects) was designed to compare the potential to repair articular cartilage of chondrocytes taken from different regions in osteoarthritic cartilage with that of mesenchymal stem cells prepared from bone marrow of the same subject.Design:Cartilage biopsies, bone marrow, and blood samples were taken from each of 30 individuals with chronic osteoarthritis (aged 62-85 years) undergoing total knee replacement. The chondrogenic potential of chondrocytes isolated from cartilage biopsies taken from different regions of osteoarthritic cartilage was compared with that of mesenchymal cells by quantitative analysis of several chondrocyte specific markers and an ex vivo cartilage differentiation assay.Results:Cartilage-derived articular chondrocytes are superior to bone marrow–derived cells when compared for their ex vivo chondrogenic potential. Interestingly, there was marked and significant difference in the expression of chondrocytic markers between chondrocytes derived from adjacent, visually distinct regions of the diseased cartilage. When cultured in the presence of a fibroblast growth factor 2 variant, all cell samples from both tissues showed a high degree of chondrogenic potential.Conclusions:Although bone marrow–derived mesenchymal cells, when supplemented with the appropriate chondrogenic factors, are a suitable source for autologous cartilage implantation, adult chondroprogenitor cells, particularly those from moderately affected regions of the osteoarthritic joints, demonstrate superior chondrogenic potential.

The 40.6-kDa ␤ subunit of DNA polymerase III of Escherichia coli is a sliding DNA clamp responsible for tethering the polymerase to DNA and endowing it with high processivity (Stukenberg, P. T., Studwell-Vaughan, P. S., and O'Donnell, M. (1991) J. Biol. Chem. 266, 11328 -11334). UV irradiation of E. coli induces a smaller 26-kDa form of the ␤ subunit, termed ␤*, that, when overproduced from a plasmid, increases UV resistance of E. coli (Skaliter, R., Paz-Elizur, T., and Livneh, Z. (1996) J. Biol. Chem. 271, 2478 -2481). Here we show that this protein is synthesized from a UV-inducible internal gene, termed dnaN*, that is located in-frame inside the coding region of dnaN, encoding the ␤ subunit. The initiation codon and the Shine-Dalgarno sequence of dnaN* were identified by site-directed mutagenesis. The dnaN* transcript was shown to be induced upon treatment with nalidixic acid, and transcriptional dnaN*-cat gene fusions were UV inducible, suggesting induction of dnaN* at the transcriptional level. Analysis of translational dnaN*-lacZ gene fusions revealed that UV induction was abolished in strains carrying the recA56, lexA3, or ⌬rpoH mutations, indicating involvement of both SOS and heat shock stress responses in the induction process. Expression of dnaN* represents a strategy of producing several proteins with related functional domains from a single gene.UV irradiation of Escherichia coli cells leads to the formation of both mutagenic and inactivating DNA lesions (1). The cells respond by an immediate arrest of DNA replication, followed by a period of extensive DNA repair, that operates to eliminate DNA damage in order to prevent replication obstacles (2). These processes are controlled primarily by the SOS stress regulon, which involves more than 20 genes that are commonly regulated by the LexA repressor and the RecA activator (3, 4). However, UV irradiation induces change also in heat shock genes (5) and other genes (6) which affect the post-UV physiology of the cell. We have previously found that the ␤ subunit of DNA polymerase III holoenzyme, the major replicase of the E. coli chromosome (7), limits the ability of the purified polymerase to replicate UV-irradiated single-stranded DNA (8). Consistent with this result, overproduction of the ␤ subunit from a plasmid caused a reduction in UV resistance and in UV mutagenesis of E. coli cells (9).This involvement of the ␤ subunit in UV irradiation effects prompted us to examine whether it may be present in a different form in UV-irradiated cells. We found that upon UV irradiation a smaller form of the ␤ subunit, termed ␤*, was induced. When overproduced from a plasmid under the inducible lac promoter, ␤* caused up to a 6-fold increase in UV resistance of E. coli cells, suggesting a role in recovery from UV damage, e.g. by involvement in DNA repair or reactivation of DNA replication (48).Smaller derivatives of proteins that are found in cells are frequently generated by proteolysis, as in the case of the mutagenesis protein UmuDЈ that is formed from UmuD by spe...