ALTERATION OF N-TERMINAL PHOSPHOESTERASE SIGNATURE MOTIFS INACTIVATESSACCHAROMYCES-CEREVISIAE MRE11

Saccharomyces cerevisiae Mre11, Rad50, and;Xrs2 function in a protein complex that is important for nonhomologous recombination, Null mutant s of MRE11, RAD50, and XRS2 are characterized by ionizing radiation se nsitivity and mitotic interhomologue hyperrecombination. We mutagenize d the four highly conserved phosphoesterase signature motifs of Mre11 to create mre11-11, mre11-2, mre11-3 and mre11-4 and assessed the func tional consequences of these mutant alleles with respect to mitotic in terhomologue recombination, chromosome loss, ionizing radiation sensit ivity, double-strand break repair, and protein interaction. We found t hat mre11 mutants that behaved as the null were sensitive to ionizing radiation and deficient in double-strand break repair. We also observe d that these null mutants exhibited a hyperrecombination phenotype in mitotic cells, consistent with previous reports, but did not exhibit a n increased frequency of chromosome loss. Differential ionizing radiat ion sensitivities among the hypomorphic mre11 alleles correlated with the trends observed in the other phenotypes examined. Two-hybrid inter action testing showed that all but one of the mre11 mutations disrupte d the Mre11-Rad50 interaction, Mutagenesis of the phosphoesterase sign atures in Mre11 thus demonstrated the importance of these conserved mo tifs for recombinational DNA repair.