The Interaction of Near U.V. (365 nm) and X-radiations on Wild-type and Repair-deficient Strains of Escherichia Coli K12: Physical and Biological Measurements

Summary

Colony-forming and single-strand break rejoining ability of a wild-type, polA and recA recB strain of E. coli K12 have been measured after treatment with a combination of 365 nm and X-radiation. Exposure to 365 nm radiation immediately before X-irradiation increased the slope of the X-ray survival curves to a maximum of approximately 3·4 times that of the untreated cells. The polA strain showed 75 per cent of this synergistic interaction, but no synergism was observed in the recA recB strain. DNA degradation, rather than strand-rejoining (measured by the alkaline sucrose gradient technique), occurred in the wild-type and repair mutant strains after similar treatment and subsequent incubation in phosphate buffer. In full medium, a limited amount of repair did occur in both the wild-type and recA recB strains during an extended incubation period, but only degradation in the polA strain. The results have been interpreted taking into account the loss of trichloracetic-acid-insoluble material from the cells (DNA degradation) during the incubation period. Although a dose of 10⁶ Jm⁻² of 365 nm radiation initially inhibits both polA (type II) and recA (type III) dependent repair of X-ray-induced single-strand breaks, only the disruption of the recA pathway (which is effectively irreversible under the set of conditions employed) ultimately affects the ability of a cell to reproduce and form a colony. The results are consistent with full medium-dependent recovery of polymerase 1 activity.