Medical Microbiology

TheMolecularBasisofBacterialGenetics 169

separatedbyintercistronicregions.Eachcistronhasitsownstartandstop

codoninthemRNA.

&Translation.Transformationofthenucleotidesequencecarriedbythe

mRNAintothepolypeptideaminoacidsequenceatthe 7 0Sribosomes.In

principle,bacterialandeukaryotictranslationisthesame.Theenzymes

andotherfactorsinvolveddo,however,differstructurallyandcantherefore

beselectivelyblockedbyantibiotics(p. 19 8ff.).

RegulationofGeneExpression...........................

Bacteriademonstrateatrulyimpressivecapacityforadaptingtotheirenvi-

ronment.Anumberofregulatorybacterialmechanismsareknown,forex-

ampleposttranslationalregulation,translationalregulation,transcription

termination,andquorumsensing(seeFig. 1. 5 ,p.20).Thedetailsofallthese

mechanismswouldexceedthescopeofthisbook.Themostimportantisreg-

ulationoftheinitiationoftranscriptionbymeansofactivationorrepression,

aprocessnotobservedinthisformineukaryotes:asinglegene,orseveral

genesinanoperonatoneDNAlocation,maybeaffected(seeFig.3. 18 ).The

mechanismthathasbeeninvestigatedmostthoroughlyistranscriptional

regulationofcatabolicandanabolicoperonsbyarepressororactivator.

BacterialOperonandRegulatorGene

5'

3'

3'

5'

mRNA

Transcription

Translation

Regulator protein Protein A Protein B

CR IR

DNA

Promoter Terminator Operator

CR = Cistron region

(coding region)

IR = Intercistron region

5' 3'

Transcription

Translation

CR

Start Stop Start Stop

Regulator gene Gene A Gene B

Effector

P T PO

P T O

T

Fig.3. 18 AnoperonisaDNAsequencethatusuallyincludesseveralstructural

genes.Thesegenescodeforproteinsthatarefunctionallyrelated.Transcription

ofanoperonisoftenactivatedorrepressedbytheproductofaregulatorgene

locatedelsewhereonthechromosome.

3

Kayser, Medical Microbiology © 2005 Thieme