Viruses of Bacteria

Viruses of Bacteria

Chapter 13

General Characteristics of Viruses

Non-living entities Not considered

organisms Can infect organisms of

every domain All lifeforms

Commonly referred to by organism they infect

General Characteristics of Viruses

Virus architecture Virus particle called virion Consists of nucleic acid

surrounded by protein coat Protein coat termed capsid

Capsid composed of capsomers Virus have different shapes

Isometric Helical Complex

Two types of virion Naked – without envelope Enveloped – surrounded by lipid

membrane

General Characteristics of Viruses

Viral geneome Contains only single

type of nucleic acid Either DNA or RNA

NEVER BOTH Can be linear or

circular Single-stranded or

double stranded

Replication cycle overview Only multiply inside

metabolizing cell Uses host machinery to

support reproduction Every virus contains

information to make viral proteins, assure replication and move in and out of host cells

Viruses live in two phases

Extracellular phase Metabolically inert

Intracellular phase Metabolically active

Virus Interactions with Host Cells

Effect on cells depends on infecting phage Some phage multiply inside cell producing

numerous progeny Termed productive cycle

Lytic cycle Phage lyse infected cell

Some phage integrate into host genome Termed latent cycle

Lysogenic state

The six stages of the lytic cycle are:

Adsorption Penetration Transcription Replication Assembly (or

maturation) Release

Virus Interactions with Host Cells

Adsorption (Attachment)

Bacterial DNA

Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.

Penetration

Penetration Entrance of the virus OR its nucleic acid in the host

cell Plant and bacteria viruses inject the nucleic acid into the

host through the cell wall Animal viruses enter the cell whole

Animal cells have no rigid cell wall Penetration of the virus is through:

Phagocytosis in which the virus is engulfed by the cell Membrane fusion occurs with enveloped viruses when the

viral envelope fuses with the plasma membrane of the host cell

Viruses enter intact but require an uncoating step to release the nucleic acid from the protein coat

Penetration

Bacterial DNA

Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.

Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.

Transcription/ReplicationLytic cycle

Transcription/Replication Duplication of viral components During replication:

Virus will inhibit activity of the host DNA Virus produces enzymes to destroy host DNA Viral DNA takes over and begins producing proteins

Early viral proteins are synthesized and are associated with the replication of viral nucleic acid

Late viral proteins are synthesized and are associated with the replication of other viral structures

Transcription/Replication

Bacterial DNA

Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.

Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.

RNA

Phage-inducedproteins

Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.

DNA

Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.

Assembly

Assembly (or maturation) This stage is the assembling of the replicated

viral components into an intact, mature virus

Assembly

Bacterial DNA

Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.

Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.

RNA

Phage-inducedproteins

Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.

DNA

Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.

Step 5: AssemblyPhage components areassembled intomature virions.

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DNA insidehead

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Release

Release Host cell bursts and releases viruses to the

outside environment Viruses are now extracellular

As virus leave the host cell the envelope is picked up

The envelope is made of a portion of the host cell plasma membrane which becomes the lipid envelope of the virus

Release

Bacterial DNA

Step 1: AttachmentThe phage attach tospecific receptors onthe cell wall of E. coli.

Step 2: PenetrationFollowing attachment, phage DNAis injected into the bacterial cell,leaving the phage coat outside.

RNA

Phage-inducedproteins

Step 3: TranscriptionPhage DNA is transcribed,producing phage mRNA, whichis translated to phage proteins.

DNA

Step 4: Replication ofPhage DNA andSynthesis of ProteinsPhage coat proteins,other proteincomponents, andDNA are producedseparately. Host DNAdegraded.

Step 5: AssemblyPhage components areassembled intomature virions.

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DNA insidehead

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Step 6: ReleaseThe bacterial celllyses and releasesmany infective phage.

Lysogeny Replication of a temperate virus

This is a non productive cycle Lysogeny begins like the lytic cycle

Adsorption Penetration, then; Incorporation http://highered.mcgraw-hill.com/sites/007255678

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Virus Interactions with Host Cells

Incorporation Viral nucleic acid incorporates onto the host chromosome

This virus is called a prophage Once incorporated, repressor genes are expressed and

repressor proteins are produced These hide or suppress the viral gene from host

immune responses The viral DNA replicated only when the host cell replicates

This allows for a population of bacterial cells that carry viruses

Cell eventually “pops” off the host chromosome and returns to the lytic cycle

Virus Interactions with Host Cells

Lysogenic conversion Prophage can confer new properties on cell Phage DNA not completely suppressed

Genes coding for trait are expressed Organism displays new trait

Streptococcus pyogenes manufactures toxin resulting in scarlet fever due to lysogenic conversion

Virus Interactions with Host Cells

Host Ranges of Phages

Number of different bacteria that phage can infected termed host range Usually limited to single bacterial species for a

single phage Factors limit host range

Two most important Phage must be able to attach to host receptors Restriction-modification system host cell must

overcome

Receptors on bacterial surface Vary in chemical structure and location

Usually on bacterial cell wall Sites can be altered by two mechanisms

Receptor sites can be altered by mutation Lysogenized bacteria can alter cell surface

Results in alteration of receptor site

Host Ranges of Phages

Host Ranges of Phages Restriction-modification system

Bacteria have two genes coding for enzymes of restriction-modification system

Restriction enzyme that codes for endonuclease

Cuts small segments of DNA

May recognize viral DNA and cut it

Modification enzyme attaches methyl group to DNA recognized by restriction enzyme

Methylated bases not recognized by restriction enzyme

Protects cells own DNA