microbio block D- bacteriophages

adsorption

adsorption

attachment of phage to bacterial surface

proteins translated as…

trimers

why trimers?

for the icosahedral structure

assembly and packaging restricted by…

geometry of the capsid

Burst size

number of virions released

What are the best studied bacteriophages?

ones that infect enteric bacteria like E.coli and salmonella

eg T4 and lambda

most bacteriophages have…

dsDNA

most bacteriophages are…

naked

some are enveloped

viruses of archaea

most resemble enteric bacteriophages

only dsDNA ones have been discovered

10^31

bacteriophages number

10^30

bacteria number

10^7

phages/ml of seawater

10^14g

mass of phages

Why are iron transport proteins a good receptor?

they indicate that the bacteria is in a favourable environments and growing well therefore the phage wants to infect this cell

What is a permissve cell?

cell that allows complete replication cycle of virus to occur

What bacteria does T4 infect?

E.coli

T4 attachment

attach via tail fibres

interact with polysaccharides on E.coli cell envelope

tail sheath contracts and viral DNA passes into cytoplasm

T4 genome

dsDNA

circularly permuted

terminally redundant

prokaryotic defence mechanisms

CRISPR

restriction modification system

restriction enzymes

restriction modification system

dna destruction

specific to defence against dsDNA viruses

restriction enzymes

cleaves DNA at specific sequences

modification of hosts own DNA at restriction enzyme recognition sites prevents cleavage of own DNA

evasion mechanisms

chemical modification of viral DNA

production of proteins that inhibit host cell restriction enzymes

T4 evasion mechanisms

restriction enzyme resistant 5-hydroxymethylcytosine

early proteins

enzymes for synthesis and glucosylation of T4 base

enzymes that function in T4 replisome

proteins that modify host RNA polymerase

middle proteins

additional proteins that modify host RNA polymerase

production of viral proteins

late proteins

virus coat

structural

synthesised in larger amounts

packaging of T4 genome

prohead assembled

packaging motor assembled

dsDNA pumped into head under pressure using ATP

head filled with DNA

tail, tail fibres and other components added

virulent mode

viruses lyse host cell after they infect

temeperate mode

viruses replicate their genomes in tandem with host genome without killing the host

can switch to lytic pathway and lyse host

lysogeny

most virus genes not expressed and virus genome (prophage) is replicated in synchrony with host chromosome

lysogen

bacterium containing a prophage

bacteriophage lambda

linear dsDNA genome

complementary single stranded regions 12 nucleotides long at 5’ terminus of each strand

lambda upon penetration

DNA ends base pair, forming cos site

DNA ligates and forms double stranded circle

when lambda is lysogenic…

its DNA becomes integrated into E.coli chromosome at lambda attachment sites

lambda attachments sites in E.coli

att lambda

steps of lambda integration

1. genome cyclises at its cohesive ends

2. a site-specific endonuclease creates staggered ends of phage and host DNA

3. lambda genome integrates to host genome and gaps are closed by DNA ligase

When lambda enters lytic pathway…

it synthesises long, linear concatemers of DNA by rolling circle replication

What controls swithc between lytic and lysogenic?

genetic switch

cro and cl

cl protein

lambda repressor

represses lambda lytic events

activates lysogenic events

cro repressor

represses lysogenic events

activated lytic events

two types of transduction

generalised

specialised

generalised transduction

dna from any portion of host genome packaged inside virion

specialised transduction

dna from specific region of host chromosome is integrated directly into virus genome