RadA/Sms is a highly conserved eubacterial protein that shares sequence similarity with both RecA strand transferase and Lon protease. We examined mutations in the radA/sms gene of Escherichia coli for effects on conjugational recombination and sensitivity to DNA-damaging agents, including UV irradiation, methyl methanesulfonate (MMS), mitomycin C, phleomycin, hydrogen peroxide, and hydroxyurea (HU). Null mutants of radA were modestly sensitive to the DNA-methylating agent MMS and to the DNA strand breakage agent phleomycin, with conjugational recombination decreased two-to threefold. We combined a radA mutation with other mutations in recombination genes, including recA, recB, recG, recJ, recQ, ruvA, and ruvC. A radA mutation was strongly synergistic with the recG Holliday junction helicase mutation, producing profound sensitivity to all DNA-damaging agents tested. Lesser synergy was noted between a mutation in radA and recJ, recQ, ruvA, ruvC, and recA for sensitivity to various genotoxins. For survival after peroxide and HU exposure, a radA mutation surprisingly suppressed the sensitivity of recA and recB mutants, suggesting that RadA may convert some forms of damage into lethal intermediates in the absence of these functions. Loss of radA enhanced the conjugational recombination deficiency conferred by mutations in Holliday junction-processing function genes, recG, ruvA, and ruvC. A radA recG ruv triple mutant had severe recombinational defects, to the low level exhibited by recA mutants. These results establish a role for RadA/Sms in recombination and recombinational repair, most likely involving the stabilization or processing of branched DNA molecules or blocked replication forks because of its genetic redundancy with RecG and RuvABC.The radA/sms gene was initially identified in a screen for radiation-sensitive mutants of Escherichia coli (13). radA mutants showed a modest decrease in survival after UV or Xirradiation exposure and in repair of DNA breaks (40). This phenotype is growth medium dependent: in minimal medium, strains are less resistant and have no radA-dependent component of survival. This may be related to the fact that in rich growth medium, E. coli cells have multiple sister chromosomes that can interact by recombination to effect repair. The sms gene was defined as an open reading frame downstream of, and coregulated with, serB, whose inactivation caused slight sensitivity to methyl methanesulfonate (MMS) (39). Later, sms and radA were identified as the same gene (46).The RadA/Sms predicted protein sequence is a composite of three characteristic regions. It contains a putative zinc finger at its N terminus with a CXXC-X n -CXXC motif. Its middle region is related to the RecA strand exchange protein and the DnaB replicative DNA helicase (24), containing both Walker A and Walker B boxes characteristic of ATPases. The highly conserved motif among prokaryotic RadA orthologs, KNRFG, is found at the C-terminal edge of this RecA-related region. The C-terminal 150 amino acids of RadA/Sms is related...