[W5] Gene Regulation in Prokaryotes Flashcards
(24 cards)
What are the two life cycles of phage lambda?
Lytic cycle: Phage replicates and lyses the host.
Lysogenic cycle: Phage DNA integrates into host genome and remains dormant.
What are the two main promoters in lambda phage?
PL: Leftward promoter
PR: Rightward promoter
What does PRM stand for, and what does it do?
Promoter for Repressor Maintenance; drives transcription of the cI repressor.
What does the cI gene encode?
Lambda repressor – maintains lysogeny by repressing PL and PR and promoting its own expression from PRM.
What does the cro gene encode?
Cro repressor – promotes lysis by repressing PRM, preventing repressor synthesis.
What is the role of the N gene product?
pN is an antiterminator that allows transcription of delayed early genes.
What is the role of the Q gene product?
pQ is another antiterminator that allows transcription of late genes in the lytic cycle.
How many operator sites are there in lambda regulation?
Six total: OR1, OR2, OR3 (near PR and PRM); OL1, OL2, OL3 (near PL).
What are operator sequences?
17 bp imperfect palindromes where regulatory proteins bind.
What does the repressor binding do at OR1 and OR2?
Represses PR (blocking lytic genes) and enhances PRM (activating repressor synthesis).
What happens when repressor binds OR3?
PRM is turned off → negative feedback on repressor expression.
What is meant by cooperative binding in lambda regulation?
Repressor binding to one operator enhances binding to adjacent ones, forming tetramers or octamers.
What is the consequence of cooperative binding?
It increases sensitivity and reduces the concentration of repressor needed for effective control.
What DNA-binding motif does the lambda repressor use?
Helix-turn-helix (HTH); interacts with the major groove of DNA.
What enhances repressor-DNA affinity?
N-terminal arms that contact the opposite face of the DNA duplex.
What does Cro use to bind DNA?
A similar HTH motif, but with different contact residues for DNA specificity.
How does Cro promote the lytic cycle?
Binds OR3, represses PRM (shuts off cI), allowing lytic genes from PR to be expressed.
How does the repressor maintain lysogeny?
Binds OR1/OR2 to shut down cro and enhance its own expression via PRM.
What triggers the switch from lysogeny to lytic cycle?
DNA damage (e.g., UV) activates RecA, which cleaves and inactivates lambda repressor.
What is the SOS response?
A cellular response to DNA damage; RecA cleaves repressors like LexA and lambda cI to enable repair (and phage induction).
What does PR control?
Expression of cro and lytic genes.
What does PRM control?
Expression of the lambda repressor (cI gene).
What is the critical decision point between lysis and lysogeny?
Whether cII expression leads to sufficient cI synthesis to suppress Cro activity.
How does cI function in autoregulation?
Enhances its own transcription from PRM and represses PR (inhibiting cro and lytic genes).
