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46 54 60 64 68 71 75 820

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Frequency

Frequency

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RNA VIRUS REPLICATION STRATEGIES

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Objectives

• Classes of RNA genomes Baltimore Classification

• Different strategies used by RNA viruses

• RNA replication and mRNA synthesis• Initiation, elongation and termination of

replication• Examples

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RNA VIRUS STRATEGIES

RNA -> RNARNA-dependent RNA polymerase

RNA -> DNARNA-dependent DNA polymerase- reverse transcriptase

Host cell DNA -> RNADNA-dependent RNA polymerase

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ALL ANIMAL RNA VIRUSES CODE FOR A POLYMERASE

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Need to make mRNA

PLUS (POSITIVE) SENSE RNA GENOMES

AAA(+ve) sense mRNA

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Need to make mRNA

MINUS (NEGATIVE) SENSE RNA GENOMES

AAA(+ve) sense mRNA

(-ve) sense genomic RNA

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Need to make mRNA

MINUS (NEGATIVE) SENSE RNA GENOMES

RNA polymerase must bepackaged in virion.

AAA(+ve) sense mRNA

(-ve) sense genomic RNAIf used, RNA modifying enzymes arepackaged in virion.

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Need to make mRNA

DOUBLE-STRANDED RNA GENOMES

AAA(+ve) sense mRNA

Double-stranded genomic RNAIf used, RNA modifying enzymes arepackaged in virion.

RNA polymerase must bepackaged in virion.

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Need to make mRNA

RETROVIRUSES

DSDNA

+VE RNA

DSDNA

Reverse transcriptase mustbe packaged in virion.

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RNA viruses that do NOT have a DNA phase

TranscriptionNon-infectiousYesDouble-stranded RNA

TranscriptionNon-infectiousYesNegative-stranded RNA

TranslationInfectiousNoPlus-stranded RNA

Initial event in cell

Infectivity of RNA

RNA-dependent RNA polymerase (=transcriptase) IN VIRION

Genome

TranscriptionNon-infectiousYesDouble-stranded RNA

TranscriptionNon-infectiousYesNegative-stranded RNA

TranslationInfectiousNoPlus-stranded RNA

Initial event in cell

Infectivity of RNA

RNA-dependent RNA polymerase (=transcriptase) IN VIRION

Genome

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Genome RNA-dependent DNA polymerase (=reverse transcriptase) in virion

Infectivity of RNA

Initial event in cell

Plus-stranded RNA Yes Non-infectious Reverse

transcription

RETROVIRUSES

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GENOME SIZE

• POLYMERASE• ATTACHMENT PROTEIN• CAPSID PROTEIN• RELATIVELY FEW OTHER PROTEINS• VIRION SIZE MAY BE QUITE LARGE

BUT LIMITED NUMBER OF PROTEINS

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The monocistronic mRNA problem

• Make one monocistronic mRNA per protein• Make a primary transcript and use alternative splicing• Make a large protein and then cut it into smaller

proteins• Include special features in the mRNA which enable

ribosomes to bind internally

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RIBOSOMES

mRNA

PROTEIN

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GENOME SIZE

• TAUTOMERIZATION• PROOF READING