Characterization of a Strand Transferase Activity from Human Cells

Abstract

In procaryotes and in lower eucaryotes, mutants deficient in homologous recombination display a high sensitivity to a variety of DNA-damaging agents. In these organisms, recombination is clearly associated with DNA repair. In E. coli, for example, the recA gene controls homologous recombination, but its expression is also required for numerous processes of recovery from DNA-damage. So far, none of the repair mutants identified in mammalian cells have been shown to be deficient in recombination. This is not altogether surprising, since attempts to merely detect recombination activities in higher eucaryotes have only recently become successful. The development of shuttle vectors and drug resistance markers has allowed the demonstration of recombination between both chromosomal and extrachromosomal genes. Most studies have monitored, in whole cells, the reconstruction of a functional gene from pairs of plasmids or viruses carrying mutant or truncated genes. The same type of investigation has recently been applied to cell extracts and revealed that, as in yeast, the introduction of double-strand breaks in plasmid DNA substrates markedly increased the frequency of exchanges. All the above studies, however, were designed for the detection of complete recombinants rather than intermediates, and provide little insight into the mechanism of the recombination process.