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Post-replication repair (PRR) is a pathway that allows cells to bypass or overcome lesions during DNA replication. In eukaryotes, damage bypass is activated by ubiquitylation of the replication clamp PCNA through components of the RAD6 pathway. Whereas monoubiquitylation of PCNA allows mutagenic translesion synthesis by damage-tolerant DNA polymerases, polyubiquitylation is required for an error-free pathway that probably involves a template switch to the undamaged sister chromatid. Both the timing of PRR events during the cell cycle and their location relative to replication forks, as well as the factors required downstream of PCNA ubiquitylation, have remained poorly characterized. Here we demonstrate that the RAD6 pathway normally operates during S phase. However, using an inducible system of DNA damage bypass in budding yeast (Saccharomyces cerevisiae), we show that the process is separable in time and space from genome replication, thus allowing direct visualization and quantification of productive PRR tracts. We found that both during and after S phase ultraviolet-radiation-induced lesions are bypassed predominantly via translesion synthesis, whereas the error-free pathway functions as a backup system. Our approach has revealed the distribution of PRR tracts in a synchronized cell population. It will allow an in-depth mechanistic analysis of how cells manage the processing of lesions to their genomes during and after replication.

Background: Ethnic differences in the association between body mass index (BMI) and body fat suggest that body composition varies across ethnic groups., Objective: To investigate the association between impedance index - a measure of tissue resistivity - and BMI in adults of different ethnic groups (Asian Indians, West Africans and White Caucasians) living in their native countries., Methods: Male (n=329) and female (n=277) adult subjects (18-50 years) living in urban areas in the UK, The Gambia and Pakistan were studied. Body weight and height were measured and BMI calculated. The same leg-to-leg bioimpedance instrument was used in each study and impedance index (height(2) (cm)/impedance (Omega)) used as measure of tissue resistivity., Results: In women, Asian Indians and West Africans had a significantly greater increase in impedance index per unit increase in BMI compared with white Caucasians (P<0.001). In men, Asian Indians had a significantly lower impedance index compared with West Africans and white Caucasians (P<0.001)., Conclusion: Different ethnic groups may have different tissue resistivity for the same BMI indicative of systematic differences in body composition.

Humans with a defect in the XPG protein suffer from xeroderma pigmentosum (XP) resulting from an inability to perform DNA nucleotide excision repair properly. Here we show that XPG makes a structure-specific endonucleolytic incision in a synthetic DNA substrate containing a duplex region and single-stranded arms. One strand of the duplex is cleaved at the border with single-stranded DNA. A cut with the same polarity is also made in a bubble structure, at the 3' side of the centrally unpaired region. Normal cell extracts introduce a nick 3' to a platinum-DNA lesion, but an XP-G cell extract is defective in making this incision. These data show that XPG has a direct role in making one of the incisions required to excise a damaged oligonucleotide, by cleaving 3' to DNA damage during nucleotide excision repair.

In human T lymphocytes the antigen receptor (Ti) is associated non-covalently on the cell surface with the invariant T3 antigen which comprises 3 chains: two glycosylated polypeptides of relative molecular mass 26,000 (Mr 26K) and 21K (gamma and delta) and one non-N-glycosylated polypeptide of Mr 19K (epsilon). The proposed function of T3 is to transduce the activation signals delivered via the antigen receptor. Recently we have shown that phorbol esters, which stimulate protein kinase C, can induce phosphorylation of the gamma subunit of the T3 antigen. But the critical question is whether T3 phosphorylation occurs as a normal consequence of immune activation of T lymphocytes. In this respect, it has been shown that immune stimulation of murine T cells results in phosphorylation of Ti-associated polypeptides that may be the functional analogues of the human T3 antigen. We have therefore monitored T3 phosphorylation after exposure of human T cells to antigen or phytohaemagglutinin (PHA). The data show that both stimuli initiate phosphorylation of the gamma subunit of the T3 antigen which indicates that T3 phosphorylation is a physiological response to immune activation.