Excision Repair Properties of Isogenic rec − Mutants of Escherichia coli K-12

Abstract

We have examined the excision repair properties of isogenic rec − and uvr − strains of Escherichia coli K-12. A recB − recC − strain excises dimers at a rate nearly that of the rec + parent, reaching the same extent of excision after a 1-hr postirradiation incubation. recA − and recA − recB − strains excise 75 to 80% of the dimers excised by their rec + parent, whereas a uvrB − strain excises no dimers during a 1-hr incubation. The doses of ultraviolet light (254 nm) required to reduce survival to 37% of the original population are 8 ergs/mm 2 for recA or recA recB mutants, 5 ergs/mm 2 for the uvrB − strain, 30 ergs/mm 2 for the recB recC mutant, and 230 ergs/mm 2 for the wild-type parent. From these data one cannot account for the ultraviolet light sensitivity of rec − strains on the basis of their excision repair properties. We conclude that rec gene products play no significant role in the early steps of excision repair. The assay we have used for excision of thymine dimers is a modification of the Carrier-Setlow technique, and is described in detail in the Appendix to this paper. To show the properties and validity of this method, results of experiments with thymine dimers formed in vitro and in vivo in E. coli K-12 are presented. These results show our method to be reproducible and sensitive to 0.005% of the total radioactive thymine present in thymine-containing dimers.