Threeflagellar genesofSalmonella typhimurium (flaAiI.2,flaQ, andflaN) werefound tobemultifunctional, eachbeingassociated withfourdistinct mutantphenotypes: nonflagellate (Fla-), paralyzed (Mot-), nonchemotactic (Che-) withclockwise motorbias, andnonchemotactic (Che-) withcounterclockwise motor bias. Thedistribution ofFla, Mot,andChemutational sites within eachgenewasexamined. Flasites were fairly broadly distributed, whereas MotandChesites were more narrowly defined. Localsubregions rich in sites ofone typewere notgenerally richinsites ofanother type. AmongChesites, there was little overlap between those corresponding toa clockwise biasandthose corresponding toa counterclockwise bias. Our results suggest thatwithin thecorresponding geneproducts there arespecialized subregions forflagellar structure, motorrotation, andcontrol ofthesenseofrotation. Thebacterial flagellum isa reversible rotary apparatus driven byprotonmotiveforce. Counterclockwise (CCW) rotation causessmooth swimming ofthebacterium, whereas an abrupt switch toclockwise (CW)rotation causestumbling. Bacteria migrate towardbeneficial environments by modulating thefrequency ofswitching betweenCCW and CW senses(see references 9and16forreviews). InSalmonella typhimurium, nearly 50genesareinvolved intheformation andfunction oftheflagella (12, 14,24). They havebeenplaced into three categories: those necessaryfor flagellar formation (fla), forflagellar rotation (mot), andfor modulation ofswitching (che). Thecorresponding mutant phenotypes are nonflagellate, paralyzed, andnonchemotactic, respectively. Twooftheflagenes,flaAII.2 andflaQ, areparticularly interesting because theygiverise toseveral typesofmutants, notonlynonflagellate (thenullphenotype), butalso paralyzed (flaAII.2) andnonchemotactic (bothflaAII.2 and flaQ) (5,7,11,12,21,25). Thegeneproducts aretherefore needed forrotation andformodulation ofswitching aswell as forflagellar structure, i.e., theycan beregarded as multifunctional. Elucidation oftheir properties andthose of any other similar geneproducts isobviously important for theunderstanding ofbacterial behavior. Within eachgene,thelocations ofmutational sites associated withthevarious phenotypes should reflect structural andfunctional features ofthegeneproduct. Inthepresent study, alarge numberofspontaneous Fla-, Mot-,andChemutantsofS.typhimuriurn were isolated. Based on deletion mutantsfromthatcollection, genes were subdivided into units we term"segments." Themutational sites-those conferring theFla-, Mot-, andChe-phenotypes-of allof
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