ANALYSIS BY PULSED-FIELD GEL-ELECTROPHORESIS OF DNA DOUBLE-STRAND BREAKAGE AND REPAIR IN DEINOCOCCUS-RADIODURANS AND A RADIOSENSITIVE MUTANT

Double-strand break (dsb) induction and rejoining after ionizing radiation was analysed in Deinococcus radiodurans and a radiosensitive mutant by pulsed-field gel electrophoresis. Following 2 kGy, migration of genomic DNA (not restriction cleaved) from the plug into the gel was extensive, but was not observed after 90 min postirradiation recovery. By this time D. radiodurans chromosomes were intact, as demonstrated by restoration of the Not I restriction cleavage pattern of 11 bands, which we found to be the characteristic pattern in unirradiated cells. Following the higher exposure of 4 kGy, dsb rejoining took approximately 180 min, twice as long as required following the 2 kGy exposure. Restoration of dsb in the radiosensitive mutant strain 112, which appears to be defective in recombination, was markedly retarded at both 2 and 4 kGy. The Not I restriction fragments of wild-type D. radiodurans and the radiosensitive mutant were identical, totaling 3.58 Mbp, equivalent to 2.36 x 10(9) daltons per chromosome.