Medical Microbiology

Replication 385

Hepadnaviruses:thegenomeconsistsofanssDNAantisensestrandanda

shortsensestrand(Fig. 7 .5e).TheinfectedcelltranscribesanRNAsense

strand(“templatestrand”)fromtheantisensestrand.Thistemplatestrand

isintegratedinviruscapsidstogetherwithanRTDNApolymerase.Thepoly-

merasesynthesizesacomplementaryantisenseDNAand,to“sealoff”the

endsofthegenome,ashortsenseDNAfromthetemplatestrand.

— RNAviruses:sinceeukaryoticcellspossessnoenzymesforRNArepli-

cation,thevirusmustsupplytheRNA-dependentRNApolymerase(s)

(“replicase”).Theseenzymesarethusinanycasevirus-codedproteins,

andinsomecasesareactuallycomponentsofthevirusparticle.

Single-strandedRNA:insense-strandviruses,theRNAfunctionsasmRNA“as

is,”meaningtheinformationcanbereadoff,andthereplicasesynthesized

immediately.Antisense-strandvirusesmustfirsttranscribetheirgenomeinto

acomplementarystrandthatcanthenactasmRNA.Inthiscase,thepoly-

meraseforthefirsttranscriptioniscontainedinthematurevirionandde-

liveredintothecell.InssRNAviruses,whethersenseorantisensestrands,

complementarystrandsofthegenomeareproducedfirst(Fig. 7 .5a,b),

thentranscribedintodaughterstrands.Theythereforeonceagainshow

thesamepolarityastheviralgenomeandareusedinassemblyofthe

newviralprogeny.

Double-strandedRNA:atranslatablesense-strandRNAisproducedfrom

thegenome,whichconsistsofseveraldsRNAsegments(segmentedgenome).

Thisstrandfunctions,atfirst,asmRNAandlaterasamatrixforsynthesisof

antisense-strandRNA(Fig. 7 .5c).Hereaswell,anRNA-dependentRNApoly-

meraseispartofthevirusparticle.

Retrovirusesalsopossessasense-orientedRNAgenome,althoughitsre-

plicationdiffersfromthatofotherRNAviruses.Thegenomeconsistsoftwo

single-strandedRNAsegmentswithsensepolarityandistranscribedbyan

enzymeinthevirion(reversetranscriptase[RT])intocomplementaryDNA.

TheDNAiscomplementedtomakedsDNAandintegratedinthecellgenome.

Transcriptionintosense-strandRNAisthebasisforbothviralmRNAandthe

genomicRNAintheviralprogeny(Fig. 7 .5d).

7

Kayser, Medical Microbiology © 2005 Thieme