Lecture 4: Bacteriophage Lambda

Vocabulary flashcards covering major proteins, promoters, regulatory elements, and life-cycle events of bacteriophage λ as discussed in Lecture 4.

Bacteriophage λ (lambda)

A temperate bacteriophage that can undergo either lytic or lysogenic life cycles in Escherichia coli.

J protein

Attachment fiber located at the end of λ’s helical tail that binds the LamB maltose transporter on E. coli.

LamB

Maltose-transport porin on the E. coli surface that serves as the receptor for λ attachment.

cos sites

Complementary cohesive ends at both termini of the λ genome that anneal to circularize the DNA after injection.

Rolling-circle replication

Replication mode used by circular λ DNA during the lytic cycle, producing concatemers for packaging.

Integration

Site-specific recombination of λ DNA into the bacterial chromosome, initiating lysogeny.

Lytic life cycle

Phage pathway involving replication, assembly, and host lysis to release progeny phage.

Lysogeny

State in which λ DNA is integrated as a prophage and replicates with the bacterial chromosome.

Prophage

Integrated λ genome within the host chromosome during lysogeny.

Induction

Stress-triggered switch from lysogeny to lytic growth, often initiated by UV-activated SOS response.

Immediate early genes

First λ genes transcribed by host RNA polymerase from PL and PR; include cro and N.

Delayed early genes

Genes expressed after antitermination by pN; encode functions for replication and regulation (e.g., cII, cIII, O, P, Q).

Late genes

Structural and lysis genes transcribed from PR′ after activation by pQ.

Transcriptional cascade

Sequential activation of λ gene sets where products of one stage regulate transcription of the next.

PL promoter

Leftward promoter active immediately after infection; drives N and other early genes.

PR promoter

Rightward promoter active immediately after infection; drives cro and other early genes.

pN (N protein)

Antiterminator that allows RNA polymerase to read through terminators and transcribe delayed early genes.

Antitermination

Mechanism by which pN (and later pQ) modify RNA polymerase to ignore transcriptional terminators.

cI repressor

Protein that binds OL and OR operators to block lytic promoters and maintain lysogeny.

cro protein

Repressor that binds OR (preferentially OR3) to inhibit cI synthesis and commit the phage to lysis.

PRE promoter

Very weak promoter upstream of cI; activated by cII to start cI transcription early in infection.

cII protein

Unstable activator that binds PRE (and PI) to promote cI transcription; stability dictates lysogeny.

cIII protein

Protease inhibitor that stabilizes cII by protecting it from host protease HflA.

HflA protease

Host ATP-dependent protease that degrades cII, favoring the lytic cycle when active.

OL operator

Left operator region where cI or cro bind to regulate PL.

OR operator

Right operator region with three sites (OR1, OR2, OR3) regulating PR and PRM via cI or cro binding.

PRM promoter

Promoter for maintenance of repressor; transcribes cI after lysogeny is established.

OR1, OR2, OR3

Three operator sites with decreasing cI affinity (OR1 > OR2 > OR3) and increasing cro affinity (OR3 > OR2 > OR1).

attP (POP′)

λ attachment site on the phage genome recognized by Int for integration.

attB (BOB′)

Bacterial chromosomal attachment site between gal and bio genes used for λ integration.

Int (integrase)

λ enzyme that mediates site-specific recombination between attP and attB.

IHF (Integration Host Factor)

Bacterial protein that bends DNA and assists Int in forming the intasome.

Intasome

Nucleoprotein complex of Int, IHF, and att sites that executes λ integration.

RecA protein

SOS–response protein that stimulates autoproteolysis of LexA and homologous cI, triggering induction.

LexA repressor

Bacterial SOS repressor homologous to cI; its cleavage derepresses DNA-repair genes.

Autoproteolysis

Self-cleavage of cI (or LexA) activated by RecA, causing release from operators.

Xis protein

λ excisionase that, with Int and IHF, promotes excision of prophage from the chromosome.

O protein

λ replication initiator that binds the origin (ori) within its own gene.

P protein

λ replication factor that recruits host helicase DnaB to the O-ori complex.

DnaB helicase

Host replication enzyme recruited by P to initiate rolling-circle synthesis of λ DNA.

Concatemer

Multimeric linear DNA produced by rolling-circle replication, later cleaved and packaged into phage heads.

pQ protein

Second antiterminator that enables transcription from late promoter PR′ for structural genes.

PR′ promoter

Late promoter activated by pQ to express capsid, tail, and lysis genes.

Plaque with halo

Plaque morphology where lysogenic bacteria at edges resist superinfection, giving fuzzy borders.