Cloning andCharacterization oftheAeromonas caviae recAGene andConstruction ofanA.caviae recAMutant

Arecombinant plasmid carrying therecAgeneofAeromonas caviae wasisolated fromanA.caviae genomic library bycomplementation ofanEscherichia coil recAmutant. Theplasmid restored resistance tobothUV irradiation andtotheDNA-damaging agent methyl methanesulfonate intheE.coli recAmutant strain. The cloned genealso restored recombination proficiency asmeasured bytheformation ofklc+ recombinants from duplicated mutantlacZgenes andbytheability topropagate astrain ofphage A(red gam)thatrequires host recombination functions forgrowth. Theapproximate location oftherecAgeneonthecloned DNAfragment wasdetermined byconstructing deletions andbytheinsertion ofTnS,bothofwhichabolished theability ofthe recombinant plasmid tocomplement theE.coli recAstrains. A.caviae recA::TniS wasintroduced into A.caviae byP1transduction. Theresulting A.caviae recAmutant strain wasconsiderably moresensitive toUV light thanwasitsparent. Southern hybridization analysis indicated that theA.caviae recAgenehasdiverged from therecAgenes fromavariety ofgram-negative bacteria, including A.hydrophila andA.sobria. Maxiceil labeling experiments revealed that theRecAprotein ofA.caviae hadanMrofabout 39,400. TherecAgeneofEscherichia coli encodes aprotein with amolecular weight ofapproximately 38,000 (2,27). This protein isessential bothtotheSOSresponse andtohomol- ogousrecombination (6,29). TheRecAprotein isalso responsible forthecleavage oftemperate bacteriophage repressors, andthusisrequired fortheinduction ofsuch phages aslambda, P22,and480(12, 18,29). Results ofin vitro studies withpurified RecAprotein havedemonstrated thatitcanperform several different enzymatic activities, including proteolysis ofLexAandbacteriophage repressors aswell asATPhydrolysis, whenitcatalyzes theearly steps ofhomologous recombination (14, 15,17,24,29). Results of recent studies havesuggested, however, that RecAmaynot beaprotease, butmayactindirectly asapositive effector of anautodigestion reaction performed byLexAandphage lambda repressors (16, 28).