Replication cycle of Viral particles

1

Bacteriophages (Phages or Bacterial Viruses)

• Obligate intracellular parasites, using eubacteria orarchaea as hosts

• Many generalities of phage lifecycle hold true forplant/animal viruses

• Different representatives known with variety ofgenomes: ssRNA, dsRNA (rare in phage), ssDNA,and dsDNA, with either linear or circular moleculespackaged into viral particles (see Tables 10.1/19.1)

• Phage genomes vary in size from 3000 bases to650kb in size

• Phage morphology varies from simple icosahedra orhelical filaments to complex tailed structures

Viral particle structureFig. 10.3

Phages typically with simple protein capsid around genome; see common phage capsid morphologies/sizes in Figs. 10.12

Enveloped viruses typical for euks

Viral particles• Typically, genomic material of phage is 25-50% of

virus mass• Usually the viral genomes tightly packed with “useful"

info (e.g., 90% of M13 genome represents proteincoding region)

• Phage particles are metabolically inactive, but theymay contain enzymes involved in virus proliferation(e.g., lysozyme or RNA/DNA polymerase)

• Specificity for host cell binding is determined byfeatures (usually proteins) on capsid

Replication cycle ofbacterial virus

Fig. 10.8

to specific surface receptor

Early viral proteins ofteninhibit host functions.

Cells usually lyse.

(or viral RNA)

Late viral proteins often act incapsid assembly and genomepackaging.

Will see temporalregulation forexpression andactivity ofdifferentviral proteins.

Different phageswill exploitdifferent hostproteins duringtheir replication.

2

Variations in genetic material andexamples of different classes of viruses

Influenza;Rabies

HIV

ReovirusPolio, Norovirus; Phage MS2

Parvoviruses; Phages M13, φX174

Herpes virus;Phages T4, λ

See Fig 10.11 & Table 10.2

Terminology for ssRNA/ssDNA viral genomes

If mRNA is: 5ʼ…GAC UCG AGC …3ʼ

+ strand DNA: 5ʼ…GAC TCG AGC …3ʼ (sense)– strand DNA: 3ʼ…CTG AGC TCG …5ʼ

(template)

+ strand RNA: 5ʼ…GAC UCG AGC …3ʼ– strand RNA: 3ʼ…CUG AGC UCG …5ʼ

mRNA is always considered to be + strand

ssRNAphage MS2

• Icosahedral capsid surrounds +strand linear genome, copied 5ʼ→3ʼ by viral RNA polymerase(replicase); no DNA intermed.

• Template for viral genome is –strand, which is also made byviral replicase

• Replicase composed of viralprotein + 3 host proteinsnormally involved in translation.

• No proofreading activity for theviral RNA polymerase

(Maturation=A protein;Replicase=RNA pol)

Fig. 19.2 Translational control in MS2

Step I: RBS for coat is alwaysopen. Translation of thisgene opens up RBS forreplicase.

Step II: Translation of replicasegene can occur when RNAstructure blocking that RBSis disrupted.

Translation of replicase allows+/- RNA strand synthesis

Step III: maturation RBS usuallyblocked by RNA folding butopen during transcription ofviral + strand

*

**

*

i/t = translation initiation/termination site* = open RBS

3

Binding of MS2 and M13 to E. coli• Filamentous ssDNA

phage M13 binds totip of pilus on F+cells

• Icosahedral MS2binds to side of pilus

• F plasmid-deficientE. coli strains areresistant to bothM13 and MS2

(Also see Fig. 19.1)

Bacteriophage M13• + strand circular

ssDNA, 6500 basesand 9-10 genes

• DNA does not formsignificant 2° struct.inside cell

• Assymetric capsid:2700 coat proteins,with distinct bindingprotein at one end

• Virus buds from hostwithout lysis/killing

Budding without cell lysisComplex phage assembly (or export) apparatus

assembles capsid proteins and DNA and allowstransit through CM and OM of cell

Fig. 19.5

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