New recAMutations ThatDissociate theVarious RecA Protein Activities inEscherichia coli Provide Evidence foran Additional RoleforRecAProtein inUV Mutagenesis

Toisolate strains with new recAmutations that differentially affect RecAprotein functions, we mutagenized invitro therecA genecarried byplasmid mini-F andthenintroduced themini-F-recA plasmid into aArecAhost thatwas lysogenicforprophage 4)80andcarried a lacduplication. Byscoring prophage induction and recombination ofthelacduplication, we isolated new recAmutations. A strain carrying mutation recA1734 (Arg-243 changed toLeu)was foundtobedeficient in+80induction butproficient inrecombination. The mutation rendered thehost notmutable byUV,eveninakxA(Def) background. Yet,therecA1734 hostbecame mutable upon introduction ofa plasmid encoding UmuD*,theactive carboxyl-terminal fragment ofUmuD. Although therecA1734 mutation permits cleavage of andLexArepressors,itrenders thehostdeficient inthe cleavage of4)80 repressorandUmuDprotein. Another strain carrying mutation recA1730 (Ser-117 changed to Phe)was foundtobeproficient in+80induction butdeficient inrecombination. Therecombination defect conferred bythemutation was partly alleviated ina cell devoid ofLexArepressor,suggesting that, when amplified, RecA1730 protein isactive inrecombination. Since LexAprotein was poorly cleaved intherecA1730 strain while phagewasinduced, we conclude that RecA1730 protein cannotspecifically mediate LexAprotein cleavage. Ourresults showthattherecA1734 andrecA1730 mutations differentially affect cleavage ofvarious substrates. TherecA1730 mutation prevented UV mutagenesis, even upon introduction intothehostofa plasmid encoding UmuD*andwas dominant overrecA+. Withrespect toother RecAfunctions, recA1730 was recessive torecA+. Thisdemonstrates thatRecAprotein hasan additional roleinmutagenesis besides mediating thecleavage ofLexAandUmuDproteins.