Sequence ofthednaBGeneofSalmonella typhimurium

ThednaBgeneofEscherichia coli encodes ahelicase thatoperates atreplication forks ofthebacterium and certain ofitsbacteriophages toproduce separated strands suitable forsubsequent usebyprimase andDNA polymerase III. Here, we present thesequenceofthednaBgeneofSalmonella typhimurium, a functionally interchangeable analog oftheE.coli dnaBgene.TheDnaBproteins ofthese twoorganisms, inferred fromthe DNA sequences,areidentical inlength andin93%ofaminoacidresidues. Extended portions oftheDnaB proteins are alsosimilar totwophage-encoded DNA replication proteins: thegene4 helicase-primase of coliphage T7and,asreported previously (H.Backhaus andJ.B.Petri, Gene32:289-303, 1984), thegene12 protein ofSalmoneUla phageP22.Incontrast, little similarity was foundbetween DnaBandeither theUvrD repair helicase or transcription termination factor Rho(anRNA-DNAhelicase). Theseresults identify S. typhimurium DnaBasamemberoftheDnaBfamily ofproteins bystructural, aswell asfunctional, criteria and provide thebasis fortheeventual identification, bymutational studies, ofresidues inDnaBcritical forits function. A common feature oftheDNA replication cycle ofEsch- erichia coli andmany ofitsbacteriophages istheformation, atan origin, ofan apparatus capable ofmigrating into unreplicated DNA,unwinding this DNA ifitisnotalready single stranded, andcatalyzing or stimulating formation of RNA primers forcomplementary-strand synthesis (8,30,40, 41). Theexactcomposition ofsuchan apparatus varies for different DNA molecules, buta crucial component inmany casesistheDnaBprotein encodedintheE.colichromo- some.Thisprotein, a hexamer ofidentical subunits (5,51), utilizes theenergyderived fromATP hydrolysis todrive strand separation, as wellas itsown translocation, ina