Medical Microbiology

ViralProteinSynthesis 387

ViralProteinSynthesis

ProductionofviralmRNA.InaDNAvirusinfection,cellularpolymerases

transcribemRNAinthenucleusofthehostcellfromoneorbothDNAstrands,

wherebytheRNAisprocessed(splicing,polyadenylation,etc.)aswithcellu-

larmRNA.Anexceptiontothisprocedureisthepoxviruses,whichusetheir

ownenzymestoreplicateinthecytoplasm.

Inviruseswithantisense-strandssRNAanddsRNAthetranscriptionofthe

genomicRNAintomRNAiscarriedoutbytheviralpolymerases,usuallywith-

outfurtherprocessingofthetranscript.

Insense-strandssRNAviruses,thegenomecanfunctiondirectlyasmRNA.

Certainviruses(arenaviruses,seep.462f.)areclassifiedas“ambisense

viruses.”Partoftheirgenomecodesinantisense(–),anotherpartinsense

(+)polarity.ProteinsaretranslatedseparatelyfromsubgenomicRNAand

theantisense-codedproteinsarenottranslateduntiltheantisensestrand

hasbeentranslatedintoasensestrand.

ViralmRNAisproducedforthetranslationprocess,basedonboththe

genomeoftheinvadingvirusandthenucleicacidalreadyreplicated.

(^3) Fig. 7. 5 Schematicdiagramofnucleicacidreplication.
aSingle-strandedRNAviruseswithsense-strandgenome:thevirus-coded
RNApolymerasetranscribestheviralgenome(+)intocomplementarystrands
(–)andtheseintonewgenomicRNA(+).Thelatteristhenintegratedintheviral
progeny.
bSingle-strandedRNAviruseswithantisense-strandgenome:theRNA
polymeraseintheviriontranscribestheviralgenome(–)intocomplementary
strands(+),whichavirus-codedpolymerasethentranscribesintonewgenomic
RNA(–).
cDouble-strandedRNAviruses:whilestillinthepartiallydecapsidatedvirus
particle,thevirus-codedpolymerasetranscribescomplementarystrands(+)
fromtheantisensestrandofthe(segmented)double-strandedviralgenome;
thesecomplementarystrandsarecomplementedtomakethenewdouble-
strandedviralgenome.
dRNAreplicationinretroviruses:thereversetranscriptase(RT)carriedby
theviriontranscribestheviralgenome(twosense-RNAstrands)intocomple-
mentaryDNA(–),whichiscomplementedtoproducedsDNAandintegrated
inthecellgenome.TheviralRNAisfirstdegraded.Cellularenzymesproduce
newgenomicRNA(+).
eDNAreplicationinhepadnaviruses:bymeansofcellulartranscription,
asense-strandRNAismadefromtheviralgenome(antisenseDNA,partially
double-stranded)andintegratedinthenewvirion,whereavirus-codedRT
producesnewgenomicDNA(–)anddestroystheRNA.
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Kayser, Medical Microbiology © 2005 Thieme