Bacteriophages Flashcards
What are 4 basic functions of a bacteriophage?
1) Protection of its DNA
2) delivery of nucleic acid inside the host cell
3) conversion of the infected cell to produce phage
4) release of phage outside the cell
What are 4 stages of phage life?
1) Infection - phage attaches to bacterium & injects DNA
2) Early development - transcription of enzymes for DNA synthesis & DNA replication
3) Late development - genomes heads and tails are made
4) Lysis - breakdown of cellular membrane & release of phage progeny
What do following terms mean?
1) Virulent
2) Temperate
3) Lysogen
4) Prophage
5) Immunity
6) Induction
1) A phage that can only enter lytic cycle
2) A phage that can choose lytic cycle or lysogeny
3) A bacterium with a phage integrated into its chromosomal DNA in lysogeny
4) A phage integrated into the bacterial chromosomal DNA
5) Resistance of a lysogen to the infection of the same virus
6) events triggering entering the lytic cycle from lysogeny
What are the environmental factors determining lysis or lysogeny?
lysis - low MOI, nutrient-rich environment
lysogeny - high MOI, nutrient-poor environment
What are the 2 types of plaques in bacterial colonies?
Clear - made up of lysed cell as a result of establishment of lytic cycle
Turbid - made up of overgrowing lysogens after the establishment of lysogeny as a result of high MOI
Describe the establishment of lytic cycle in lambda.
1) Transcription of early genes is initiated by bacterial RNA polymerase
- bidirectional start from Pl and Pr –> transcription of N (leftward) and cro (rightward)
- N is an antiterminator that allows the polymerase to proceed further into the transcription of delayed early genes.
2) Delayed genes are transcribed including genes for replication enzymes and Q protein
- Q is an antiterminator allowing to proceed to transcription of late genes
3) Late genes are transcribed including all the structural genes for heads and tails
How does rolling circle replication work?
The coding strand of a circular plasmid is nicked at the origin of replication and the 5’ end is pulled away, while the new DNA is synthesized on a template strand at 3’end elongating the coding DNA strand. The final product is cleaved into multiple copies of the coding strand.
What is the name and location of the gene coding for lambda repressor protein and how are its promoters called?
cI - a gene located between promoters Pl and Pr
- it has 2 promoter regions: Prm - maintenance promoter & Pre - establishment promoter
What is the function of the lambda repressor?
Lambda repressor binds to the operators OL and OR, blocking the transcription of the early genes and stopping entering the lytic cycle. Once the lysogeny is established it maintains it by constantly occupying the operators.
What are operators? What are their structure and function? Which sites of OL and OR are preferred by which proteins?
Operators are sequences overlapping with promoters that function as binding sites for regulating factors like cro and cI. Each operon has 3 sites and 6 half-sites. cro has the highest affinity to OR3 and OL3 and cI has the highest affinity to OR1 and OL1.
What is a structure of functional repressor protein?
Dimeric cI
What gives the lysogen the immunity to the further infection by a phage from the same species?
A lysogen contains high conc of cI which upon entering of another phage to the infected cell will immediately bind to the operators OR and OL blocking any potential development.
Describe entry into lysogeny.
In delayed early transcription phase of a phage lytic cycle, 3 genes are transcribed simultaneously. cII (binds to Pre allowing RNA polymerase its recognition and initiation of transcription), cIII (prevents cII from degradation by HflA) and cro (a blocker of Prm cI promoter). Lysogeny is established when the expression of cI overcomes the expression of cro. Finally, phage Dna is integrated into the chromosome.
Describe the process of phage insertion into bacterial DNA. What cofactors are required for each stage?
The phage is integrated into the bacterial DNA by recombination between attP (POP’ sequence) and attB (BOB’ sequence) yielding BOP’ and POB’ sequences. Recombination and cross-linking of complementary sequences requires Int and IHF. Linearisation of phage and integration into the DNA structure requires Int, Xis and IHT.
