Separate Branches of the uvr Gene-Dependent Excision Repair Process in Ultraviolet-Irradiated Escherichia coli K-12 Cells; Their Dependence upon Growth Medium and the polA, recA, recB, and exrA Genes

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The extent of repair of single-strand breaks (incision breaks) induced in the deoxyribonucleic acid (DNA) of Escherichia coli K-12 cells by the uvr gene-dependent excision repair process after ultraviolet (UV) radiation was determined in the wild-type, polA1, recA56, recB21, and exrA strains. The wild-type strain repaired all incision breaks after incident doses of UV radiation (254 nm) of ∼60 J m−2 or less when incubated in growth medium, or ∼15 J m−2 or less when incubated in buffer. The polA1 strain repaired the incision breaks completely after incident doses of ∼12 J m−2 or less when incubated in growth medium, or after ∼4 J m−2 when incubated in buffer. The recA13, recB21, and exrA strains showed essentially complete repair after incident doses of 10 to 15 J m−2 whether the cells were incubated in buffer or growth medium. These results suggest that the uvr gene-dependent excision repair process may be divided into two branches, one which is dependent on the presence of growth medium and also the rec+exr+ genotype, and a second which can occur in buffer (growth medium-independent) and is largely dependent on DNA polymerase I. The presence of chloramphenicol in the growth medium resulted in an inhibition of the growth medium-dependent repair occurring in wild-type and polA1 cells and had little or no effect on the extent of repair observed in recA56, recB21, or exrA cells. The similarities between the growth medium-dependent and -independent branches of excision repair and two known processes for the repair of X-ray-induced single-strand breaks are discussed.

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