Lecture 16

Question 1

What is a virus?

Answer 1

An infectious agent that must grow/reproduce inside a host cell

• > considered obligate intracellular parasites
• > composed of nucleic acid genetic material and protein coat

Question 2

What is a bacteriophage?

Answer 2

A virus that infects bacterial cells

Question 3

Describe the shape of Phage Lambda

Answer 3

• A double stranded DNA genome

- Long flexible tail and icosahedral head

Question 4

What is a temperate phage?

Answer 4

It can choose between lytic or lysogenic pathway

Question 5

What does the lifecycle decision of the temperate phage depend on?

Answer 5

The state of the host bacterium

Question 6

Define lytic

Answer 6

The lifecycle resulting in bacterial cell lysis upon release of progeny phage

Question 7

Define lysogenic

Answer 7

The lifecycle resulting in stable carriage of the phage (prophage) within the host cell (lysogen)

Question 8

What is a virulent phage?

Answer 8

A phage that’s only able to undergo replication via the lytic cycle

Question 9

What is a lysogen?

Answer 9

A host cell harbouring a prophage during lysogeny

Question 10

What is a prophage?

Answer 10

Latent form of a temperate phage that remains within the lysogen (i.e is integrated into the host chromosome)

Question 11

Describe the genome in the phage head

Answer 11

Linear with staggered ends which give a ssDNA region of complementarity allowing DNA to stick together

Question 12

Following insertion what happens to the lambda genome?

Answer 12

The genome circularises using cohesive end sites (cos sites)

Question 13

Give the overview for the Lytic cycle

Answer 13

(Phage assembly and release)

1. Head and tail proteins synthesised
2. DNA packed into heads
3. Tails added
4. Host is lysed releasing the new phage

Question 14

Give the overview for the Lysogenic cycle

Answer 14

(phage integration and maintenance)

1. Lambda integrates
2. Prophage is stably maintained
3. Prophage passed to daughter cells

Question 15

Which of the lifecycles is considered default?

Answer 15

The lytic pathway

Question 16

How does integration occur (lysogeny) ?

Answer 16

• Occurs between two sites
  1. attP (phage)
  2. attB (bacteria)
• Site specific recombination requiring the enzyme integrase, (rather than using general homology it uses these sites)

Question 17

Is the expression pathway shared up until the decision point?

Answer 17

Yes, the same patterns for very early and early gene expression are shared and following the decision -> gene expression patterns differ

Question 18

Describe the Very Early Gene expression events

Answer 18

Following injection, the host RNA polymerase transcribes from 2 promotors

1. PL = produces N
2. PR = produces Cro

Question 19

What is Cro?

Answer 19

• Cro encodes the Cro protein
• Expressed early in infection and is a major player in establishing lytic growth
• Is a dominant protein

Question 20

What does N encode?

Answer 20

• An anti-terminator protein that enable transcription past 2 terminators resulting in expression
• > enables RNA pol to ignore transcription termination signals
• Binds to nut site (N utilisation sites)

Question 21

What is the effect of Cro?

Answer 21

Is a DNA-binding protein that REPRESSES transcription

->promotes the lytic pathway by switching transcription off at certain times

Question 22

What is the effect of CII?

Answer 22

Is a DNA-binding protein that ACTIVATES transcription

->promotes the lysogenic cycle

Question 23

What is the effect of CI?

Answer 23

A DNA binding protein that can ACTIVATE OR REPRESS transcription

• > CI activates it’s own expression
• > represses genes required for lytic cycle
• > maintains lysogeny

Question 24

How is the decision of lytic/lysogenic pathway made?

Answer 24

Through relative concentrations of CII and Cro

Question 25

In the 'decision', what is the deciding protein?

Answer 25

CII

Question 26

What would happen for the lytic pathway to be chosen?

Answer 26

A healthy cell produces lots of proteases which degrade CII, once this occurs the LYTIC pathway takes place

Question 27

Describe late lytic events

Answer 27

- Cro = represses expression of CI, all other genes, then itself and the replication genes - Q = an anti-terminator that enables expression of late lytic genes (head, tail, lysis genes)

Question 28

What would happen for the lysogenic pathway to be chosen?

Answer 28

Starved cells produce less proteases therefore CII remains intact and stable

Question 29

Describe late lysogenic events

Answer 29

CII is an activator protein that turns on: 1. Intergrase resulting in lambda integration into host 2. CI repressor which activates its own expression and binds to OL and OR to repress all other phage genes - >maintains prophage state

Question 30

Describe maintenance of lysogeny

Answer 30

- Maintained as a prophage by CI - CI is a repressor of all phage genes but is an activator of itself so keeps the phage genome 'silent' in bacterial chromosome until induction - >keeps fitness cots low for E.Coli cell - CII activates integrase and CI, which cause integration of phage and maintenance of lysogeny (respectively)

Question 31

What is PRE?

Answer 31

- Promotor for repressor establishment | - Establishes expression of CI and ensures the phage is silent