Antagonism of ultraviolet-light mutagenesis by the methyl-directed mismatch-repair system of Escherichia coli

Previous studies have demonstrated that the Escherichia coli MutHLS mismatc h-repair system can process UV-irradiated DNA in vivo and that the human MS H2.MSH6 mismatch-repair protein binds more strongly in vitro to photoproduc t/base mismatches than to "matched" photoproducts in DNA. We tested the hyp othesis that mismatch repair directed against incorrect bases opposite phot oproducts might reduce UV mutagenesis, using two alleles at E. coli lacZ co don 461, which revert, respectively, via CCC --> CTC and CTT --> CTC transi tions. F' lacZ targets were mated from mut(+) donors into mutH, mutL, or mu tS recipients, once cells were at substantial densities, to minimize sponta neous mutation prior to irradiation. In umu(+) mut(+) recipients, a range o f UV fluences induced lac(+) revertant frequencies of 4-25 x 10(-8); these frequencies were consistently 2-fold higher in mutH, mutL, or mutS recipien ts. Since this effect on mutation frequency was unaltered by an Mfd(-) defe ct, it appears not to involve transcription-coupled excision repair. In mut (+) umuC122::Tn5 bacteria, UV mutagenesis (at 60 J/m(2)) was very low, but mutH or mutL or mutS mutations increased reversion of both lacZ alleles rou ghly 25-fold, to 5-10 x 10(-8). Thus, at UV doses too low to induce SOS fun ctions, such as Umu(2)'D, most incorrect bases opposite occasional photopro ducts may be removed by mismatch repair, whereas in heavily irradiated (SOS -induced) cells, mismatch repair may only correct some photoproduct/base mi smatches, so UV mutagenesis remains susbstantial.