Excision-repair capacity of UV-irradiated strains of Escherichia coli and Streptococcus pneumoniae, estimated by plasmid recovery.

Although the biological role of many bacterial repair genes is known, there is still an interest in evaluating the capacity of repair pyrimidine dimers in some strains. For this purpose, we have developed a rapid assay. Cells bearing a plasmid are UV irradiated and incubated to allow recovery. The plasmid DNA is extracted, purified and treated with UV endonuclease from Micrococcus luteus that specifically produces single strand breaks at the site of pyrimidine dimers. The amount of open circular and covalently closed circular forms of the plasmid DNA after treatment and post-incubation provides an estimate of the repair capability of the host strain. The wild type strain and the uvrA mutant of Escherichia coli were used to adjust the assay. The lexA mutant of E. coli has been tested and its repair capability is equivalent to that of wild-type strain. The assay has been extended to Streptococcus pneumoniae, which is naturally deficient in photoreactivation and SOS-like functions. This strain is efficient in the repair of pyrimidine dimers, formed after UV irradiation.