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.

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 approximately 60 J m(-2) or less when incubated in growth medium, or approximately 15 J m(-2) or less when incubated in buffer. The polA1 strain repaired the incision breaks completely after incident doses of approximately 12 J m(-2) or less when incubated in growth medium, or after approximately 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.