Genet 270 - topic 3 lambda phage 1

what are the two alternate life cycles of temperate phages?

lytic and lysogenic

what do virulent phages do?

live only lytically

what do temperate phages do?

can exist in either lytic or lysogenic states

what is the structure of lambda DNA?

linear, ds, with 5” cohesive ends

how does lambda DNA circularize in the host?

cohesive ends base-pair and are sealed by host DNA ligase

why is circularization important?

makes the late gene cluster continuous → ensures proper head and tail gene expression

what is the timeline of lytic gene expression?

t=0: adsorption, DNA injection

t=3: early mRNA (regulators)

t=5: delayed early mRNA (DNA replication, recombination)

t=6: DNA replication begins

t=9: late mRNA (head and tail genes)

t=10: structural proteins made

t=22: first phage particle complete

t=45: lysis of host cell and release of phages

how do phages T7 and T4 differ in regulation?

T7 = synthesizes new phage RNAP

T4 = modifies host RNAP to recognize phage promoters

what is the role of N protein?

anti terminator → modifies RNAP to read through terminators → delayed early gene expression

what is the role of Cro protein?

repressor - binds of oL and oR → blocks pL and pR → prevents lysogenic genes (cl) and limits DNA replication genes

what is the role of Q proteins?

anti terminator - interacts with qut sites → ensures late gene expression (head, tail, lysis)

what is immunity in lysogens?

lysogens cannot be superinfected → due to Cl repressor blocking lytic genes

what causes plaque mutants?

mutations in cI, cII, or cIII → no lysogeny, only lytic growth

what is the role of cI, cII, cIII?

cI = repressor

cII = activator of cI transcription (pRE)
cIII = stabilizes cII

how is lysogeny estabilished?

burst of CI synthesis via cII activation of pRE

how is lysogeny maintained?

CI activates pRM promoter → continuous Cl production

how is Cl autoregulated?

low Cl = binds oR1 and oR2 → activates pRM

high Cl = binds oR1, oR2, oR3 → represses pRM

what determines the decision (lytic or lysogenic)?

balance of Cl vs. Cro binding at oL & oR

what happens if Cl wins?

lysogeny - repression of lytic genes

what happens if Cro wins?

lytic growth - repression of cl

what role does cII play?

critical determinant → stimulates cl transcription from pRE

Cl binding order at operators?

oL1 > oL2 > oL3 > ; oR1 > oR2 > oR3 (outside in)

Cro binding order at opertors?

oR3 > oR2 > oR1 ; oL3 > oL2 > oL1 (inside out)

what is the effect of Cro binding oR3?

prevents pRM activation → no Cl accumulation → lytic genes expressed