AN ESCHERICHIA-COLI STRAIN DEFICIENT FOR BOTH EXONUCLEASE-V AND DEOXYCYTIDINE TRIPHOSPHATE DEAMINASE SHOWS ENHANCED SENSITIVITY TO IONIZING-RADIATION

An Escherichia coli mutant lacking deoxycytidine triphosphate deaminas e (Dcd) activity and an unknown function encoded by a gene designated ior exhibits sensitivity to ionizing radiation whereas dcd mutants the mselves are not sensitive. A DNA fragment from an E. coli genomic libr ary that restores the wild type level of UV and gamma ray resistance t o this mutant has been cloned in the multicopy vector pBR322. Comparis on of its restriction map with the physical map of the E. coli chromos ome revealed complete identity to the recBD genes. ior affects ATP-dep endent exonuclease activity, suggesting that it is an allele of recB. This mutation alone does not confer sensitivity to UV and gamma radiat ion, indicating that lack of Dcd activity is also required for express ion of radiation sensitivity.