The malQ gene is essential for starch metabolism in Streptococcus mutans

摘要

Background:ThemalQandglgPgenes,respectively,annotatedasputative4-a-glucanotransferaseandputativeglycogenphosphorylasearelocatedwitha29nucleotideoverlapontheStreptococcusmutansgenome.WefoundthattheglgPgeneofthisorganismwasinducedwithmaltose,andthegenelikelyconstitutedanoperonwiththeupstreamgenemalQ.ThisputativeoperonwasnegativelyregulatedwiththemalRgenelocatedupstreamfromthemalQgeneandaMalR-bindingconsensussequencewasfoundupstreamofthemalQgene.S.mutansisnotabletocatabolizestarch.However,thisorganismutilizesmaltosedegradedfromstarchinthepresenceofsalivaamylase.Therefore,wehypothesizedthattheMalQ/GlgPsystemmayparticipateinthemetabolismofstarch-degradationproducts.Methods:ADNAfragmentamplifiedfromthemalQorglgPgeneoverexpressedHis-taggedproteinswiththeplasmidpBAD/HisA.S.mutansmalQand/orglgPmutantswerealsoconstructed.Purifiedproteinswereassayedforglucose-releasingandphosphorylaseactivitieswithappropriatebufferscontainingmaltose,maltotriose,maltodextrin,oramylodextrinasasubstrate,andwerephotometricallyassayedwithaglucose-6-phosphatedehydrogenaseNADPsystem.Results:PurifiedMalQproteinreleasedglucosefrommaltoseandmaltotriosebutdidnotfromeithermaltodextrinoramylodextrin.ThepurifiedGlgPproteindidnotexhibitaphosphorylasereactionwithmaltoseormaltotriosebutgeneratedglucose-1-phosphatefrommaltodextrinandamylodextrin.However,theGlgPproteinreleasedglucose-1-phosphatefrommaltoseandmaltotrioseinthepresenceoftheMalQprotein.Inaddition,theMalQenzymeactivitywithmaltosereleasednotonlyglucosebutalsoproducedmaltooligosaccharidesassubstratesfortheGlgPprotein.Conclusion:TheseresultssuggestthatthemalQgeneencodes4-a-glucanotransferasebutnota-1,4-glucosidaseactivity.ThemalQmutantcouldnotgrowinthepresenceofmaltoseasacarbonsource,whichsuggeststhatthemalQgeneisessentialfortheutilizationofstarch-degradationproducts.Keywords:malR;glgP;maltose;maltooligosaccharide;glucanotransferase;phosphorylase;glucose-releasingactivityts.(Published:6August2013)Citation:JournalofOralMicrobiology2013,5:21285-http://dx.doi.org/10.3402/jom.v5i0.21285