Molecular Chapter 27

A virus that infects bacteria

The process by which a phage infects, reproduces w/in, then kills its host

The process by which a phage infects bacteria and inserts its genome into the host genome

The latent form of the phage genome w/in bacteria

Phages that follow a lysogenic pathway

The process by which a prophage is freed form the restriction of lysogeny

The process by which prophage is incorporated into the bacterial genome

• 1. Infection - Phage attaches to bacterium
• 2. DNA - injected into bacterium
• 3. Early Development - Enzymes for DNA synthesis are made and replication begins
• 4. Late Development - Genomes, heads, and tails are made and DNA is packaged into assembled virus
• 5. Lysis - Cell is broken to release progeny phages

• 1. Early - phage genes are transcribed by host RNA pol
• 2. Middle - early product causes transcription of middle genes
• 3. Late - middle produce causes transcription of late genes

• 1. At transcription initiation - early genes are switched off when middle genes are transcribed
• 2. At transcription termination - early genes are expressed together with late genes
• 3. A phage can synthesize a new sigma factor to replace the host sigma factor or synthesize a new RNA pol

• 1. Lambda has 2 immediate early genes, N and cro
• 2. pN is an antitermination factor that allow RNA pol to continue transcription of the delayed early genes
• 3. pQ is the product of a delayed early gene and is an antiterminator that allows RNA pol to transcribe the late genes

• 1. cIII - Positive regulator
• 2. N - Antiterminator
• 3. cI - Repressor
• 4. cro - Antirepressor
• 5 cII - Positive regulator

• 1. Repressor acts at the left operator and right operator to prevent transcription of the immediate early genes (N and cro)
• 2. It also acts at the promoter P_RM to activate transcription by RNA pol of its own gene

If a second lambda phage DNA enters a lysogenic cell, repressor protein synthesized from the resident prophage genome will immediately bind the O_L and O_R in the new genome, preventing it from entering the lytic cycle

A mutation that prevents the repressor from binding the operators and sending the virus into the lytic pathway

• 1. N-terminal and C-terminal domains connected by a connector
• 2. The C-terminal domains form dimers and the N-terminal domains bind DNA

• 1. This is the motif in the N-terminal region of the lambda repressor protein
• 2. It allows the domain to fit into successive major groves of DNA

• 1. Helix-3 of each monomer lies in the wide groove on the same face of DNA and helix-2 lies across the groove
• 2. The operators recognize the aa sequence of helix-3
• 3. The N-terminal domains extend like arms around the back of the DNA strand and bind to G residues in the major groove and the phosphate backbone, contributing heavily to the binding affinity by ~1000 fold

• 1. The DNA-binding region of repressor at O_R2 contacts RNA pol and stabilizes its binding to P_RM (Autoregulatory control)
• 2. Repressor binding at O_L blocks transcription of N from P_L
• 3. Repressor binding at O_R blocks transcription of cro, but also is required for transcription of cI
• 4. Repressor binding to the operators therefore simultaneously blocks entry to the lytic cycle and promotes it own synthesis

• 1. Repressor dimers at O_L1 and O_L1 interact with dimers bound at O_R1 and O_R2 to form octamers
• 2. This brings O_L3 into proximity to O_R3 where repressors and bind and interact and restrict cI transcription

• 1. They are necessary for RNA pol to initiate transcription at the promoter P_RE
• 2. cII acts directly at the promoter and cIII protects cII from degradation
• 3. Transcription from P_RE leads to synthesis of repressor and also blocks the transcription of cro

• 1. P_RE has atypical sequences and -10 and -35
• 2. RNA pol binds the promoter only in the presence of cII
• 3. cII binds to sequences close to the -35

• Lysogeny
• 1. cII and cIII cause repressor synthesis to establish and also trigger inhibition of late gene transcription
• 2. Establishment of repressor turns off immediate and delayed early gene expression
• 3. Repressor turns on the maintenance circuit for its own synthesis
• 4. Lambda DNA is integrated into the bacterial genome at the final stage in establishing lysogeny

• Lytic
• 1. Cro binds tot he same operators as teh lambda repressor, but w/different affinities
• 2. When Cro binds to OR3, it prevents RNA pol from binding to P_RM and blocks the maintenance of repressor promoter
• 3. When Cro binds to other operators at O_R or O_L, it prevents RNA pol from expressing immediate early genes, which indirectly blocks repressor establishment

• Lysogeny vs Lytic
• 1. The delayed early stage when both Cro and repressor are being expressed is common to both cycles
• 2. The critical event is whether cII causes sufficient synthesis of repressor to overcome the action of Cro