Medical Microbiology

170 3 GeneralBacteriology

TranscriptionalRegulationofanOperon:

&Catabolicoperonshavegenesthatcodeforenzymesofcatabolicmetabolism.

Anabolicoperonscodeforenzymesofanabolicmetabolism.

&Regulators.Codeforproteinsthatcanrepressoractivatetranscriptionbybind-

ingtotheoperatororpromoterofanoperon.

&Effectors.Low-molecular-weightsignalmoleculesfromtheimmediateenviron-

mentofthebacterialcell.Canactivate(=corepressor)orinactivate(=inducer)the

repressorbymeansofanallostericeffect.

&Inductionofacatabolicoperon.Theeffectormoleculeisanutrientsubstrate

thatisbrokendownbytheproductsoftheoperongenes(e.g.,lactose).Lactose

inactivatestherepressor,initializingtranscriptionofthegenesforb-galactosidase

andb-galactosidepermeaseinthelactoseoperon.Thesegenesarenormallynotread

offbecausetherepressorisboundtotheoperator.Thecellisnotinducedtopro-

ducethenecessarycatabolicenzymesuntilthenutrientsubstrateispresent.

&Repressionofananabolicoperon.Thesignalmoleculeisthefinalproductof

ananabolicprocess,forinstanceanaminoacid.Ifthisacidispresentintheme-

dium,itcanbeobtainedfromthereandthecellneednotsynthesizetheanabolic

enzymesitwouldrequiretoproduceit.Insuchacase,bindingtotheeffectoris

whatturnstheregulatorproteinintoanactiverepressor.

Asingleregulatorproteincanalsoactivateorrepressseveralgenesnot

integratedinanoperon,i.e.,atvariouslocationsontheDNA.Suchfunctional

genegroupsarecalledregulons.Alternativesigmafactors(seep. 16 8)may

beinvolvedinthetranscriptionalactivationofspecialgeneswithspecialpro-

moters.Physiologicalcellstatusdetermineswhetherornotthesealternative

factorsareproduced.

TheGeneticVariabilityofBacteria................................

&ChangesinbacterialDNAaretheresultofspontaneousmutationsinin-

dividualgenesaswellasrecombinationprocessesresultinginnewgenesor

geneticcombinations.Basedonthemolecularmechanismsinvolved,bacte-

rialrecombinationsareclassifiedashomologous,site-specific,andtranspo-

sitional.Thelattertwoinparticularreflectthehighlevelofmobilityofmany

genesandhavemadeessentialcontributionstotheevolutionofbacteria.

Althoughsexualheredityisunknowninbacteria,theydomakeuseofthe

mechanismsofintercellulartransferofgenomicmaterialknownasparasex-

ualprocesses.TransformationdesignatestransferofDNAthatisessentially

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Kayser, Medical Microbiology © 2005 Thieme