Cross-talk between DNA repaif and recombination : two kinds of mechanisms that are mediated by DNA end-joining

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We analysed mechanisms of illegitimate recombination and its relationship with mechanisms of DNA repair, using newly developed assay systems for illegitimate recombination in Escherichia coli, budding yeast, fission yeast and mouse. Based on the analyses of E.coil DNA gyrase mutants that confer a hyper-recombination phenotype, we showed that several α-helix structures of DNA gyrase plays an important role in illegitimate recombination. It is also shown that E.coil DNA binding protein HU has an activity that suppress DNA gyrasemediated illegitimate recombination, thus playing a role in regulation of the recombination. We have also found that DnaB helicase, exonuclease VIII (RecE), RecT, and DNA ligase are involved in DNA damage-induced illegitimate recombmation in E. coli. Furthermore, many DNA repair enzymes UvrA, UvrB, MutM, Tag, AIkA, ExoI, H-NS, and StpA play important roles for suppression of DNA damage-induced illegitimate recombination. In Saccharomyces cerevisiae, it was found that a DNA topoisomemse II inhibitor induces illegitimate recombination, suggesting that DNA topoisomemse II promotes DNA double-strand bieak and then illegitimate recombination throgh DNA end-joining. It is, on the other hand, shown that Sgsl protein suppresses spontaneous illegitimate recombination. Human BLM and WRN proteins also suppressed illegitimate recombination in budding yeast. In addition, we showed that budding yeast Sccl and fission yeast Dhpl proteins are involved in DNA repair and chromosome segregation and that budding yeast Whip and fission yeast Mogi and Hskl kinase interact with Sgs 1 and Sec 1, respectively. Based on these results, we proposed several models for illegitimate recombination in E.coil and yeast.