Isolation and preliminary characterization of Escherichia coli mutants deficient in exonuclease VII

Strains of Escherichia coli containing reduced levels of exonuclease VII activity due to mutations in the xseB gene have been isolated after mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Seven mutants of independent origin deficient in exonuclease VII activity were obtained. Four of these contained defects in xseA, a locus which has been previously identified, and three others contained mutations in a gene distinct from xseA, which we have designated xseB. Genetic mapping studies place the xseB locus between proC and dnaZ. Exonuclease VII purified from KLC835 (xseA⁺ xseB3) is more heat labile than enzyme purified from the parent strain PA610 (xse⁺), showing that xseB is a structural gene for exonuclease VII. The isolation of λ transducing phage carrying xseA is also described.