EXONUCLEASE-I OF SACCHAROMYCES-CEREVISIAE FUNCTIONS IN MITOTIC RECOMBINATION IN-VIVO AND IN-VITRO

We previously described a 5'-3' exonuclease required for recombination in vitro between linear DNA molecules with overlapping homologous end s. This exonuclease, referred to as exonuclease I (Exo I), has been pu rified more than 300-fold from vegetatively grown cells and copurifies with a 42-kDa polypeptide. The activity is nonprocessive and acts pre ferentially on double-stranded DNA. The biochemical properties are qui te similar to those of Schizosaccharomyces pombe fro I. Extracts prepa red from cells containing a mutation of the Saccharomyces cerevisiae E XO1 gene, a homolog of S. pombe exo1, had decreased in vitro recombina tion activity and when fractionated were found to lack the peak of act ivity corresponding to the 5'-3' exonuclease. The role of EXO1 on reco mbination in vivo was determined by measuring the rate of recombinatio n in an exo1 strain containing a direct duplication of mutant ade2 gen es and was reduced sixfold. These results indicate that EXO1 is requir ed for recombination in vivo and in vitro in addition to its previousl y identified role in mismatch repair.