Lecture 17- Bacteriophage Lambda I Flashcards
Why is the lambda bacteriophage one of the best studied of all viruses?
Because it is a temperate phage (can do lytic cycle or latent state)
The Lambda virion
Diameter of head: 55 nm Length of tail: 135 nm Tailed bacteriophage Tailed fiber down at the tip Protein at the tip of the tail DNA compacted A LOT Length of DNA 300x diameter of head Genome: ds linear DNA, 48,514 bp; cohesive ends Length: 17 micrometers (large number) Relatively large genome
Episome:
A genetic element that can either replicate as an extrachromosomal piece of DNA or after integration into the cellular chromosome
The two prototype episomes are the F plasmid and phage lambda
Can replicate like a plasmid or not- can piggy back onto replication of chromosome of cell. episome can have a plasmid state; not the case that all plasmids are episomes. Only plasmids that can integrate into cellular chromosome are properly called episomes.
Plasmids that don’t integrate called plasmids.
Bacteriophage lambda life cycle
Injection occurs: in less than 2 min, ds DNA circularizes.
about 10 minutes before decision is made
can either go to productive response: lysis. this predominates in rich medium
or can go non productive response: lysogen. dna integrates into genome and you get a prophage
this predominates if starved
induction of lysogen occurs eventually (can be stimulated by UV light) which sends back into productive response
Never have 100% productive or non productive response
only have 90% at best or worst
Once integrated phage dna is integrated, called a prophage (only refers to phage dna within the genome)
Maintenance of lysogenic state
Repressor protein binds to two operators on prophage DNA
Lysogen refers to entire cell in nonproductive response; prophage refers to phage dna incorporated into the genome
additional features:
1) Lysogens are “immune” to superinfection
2) lambda forms turbid plaques
3) it is possible to isolate clear plaques mutants (C1, C2, C3)
4) inactivation of repressor results in induction of lysogen (activation of Rec A protein leads to proteolytic cleavage of lambda repressor protein)
Repressor protein responsible for turning off all phage genes- if not, you’d produce everything for producing more viruses
phage codes for repressor protein that binds two operators- nothing coded except for this gene
lysogens grow as little mini colonies inside the plaque
a turbid plaque = bacteria growing in the middle of the plaque. These bacteria are lysogens
another phage can’t infect cell because repressor genes are acting, lots of repressor proteins in lysogen
another lambda could not infect lysogen BUT another phage could infect
clear plaque would be completely clear- all of inside of plaque would have dead cells growing there
can from mutants that only form clear plaques (c1 2 and 3- gene for repressor protein)
Rec a is a protein involved in repair after DNA damage- presence of rec a, lambda protein itself acts as a protease to cleave and activate itself
Schedule of events in a lambda lytic infection
0-2 minutes: DNA entry and circularization, IE transcription pattern
2-10 min: theta DNA replication (origin in O gene), DE period
10-45 minutes (lysis): rolling circle replication, Late transcription (lysis occurs at about 45 min or so)
Lambda genetic map
Two principles govern the arrangement of genes on the genome
1) Genes are clustered according to function (Structural genes with structural genes, etc.)
2) Genes are located on the genome close to the site of action of the gene product (C1 gene codes for C1 repressor, operator to which C1 binds is right next to it)
Recombination- can be shuffled up among different phages- leads to evolution
Structural genes in one area (head and then tail genes) Then recombination, regulation, replication, and lysis
These principles apply to other phages and animal viruses
