phage development Flashcards
what is bacteriophage lambda
it is a phage which predates E. coli. it has 20 faces and encodes around 50 genes on 7 operons
what is the lytic phase
active replication where around 100 new phage are produced and the E.coli host is lysed
what is the lysogenic phase
integration of the phage genome into the host. it can re-enter the lytic cycle under the influence of UV
which phase is favoured by which environment
is rapidly growing E.coli the lytic phase is favoured, in weak E.coli populations the lysogenic is favoured as the bacteria can’t support a growing phage population
why does DNA circularise upon injection in the lytic cycle
to protect it from host exonucleases. it also brings together certain genes for coordinated expression
what part of the phage genome is transcribed first, in the very early stage, and by what
the N and Cro regions are transcribed first by PL and PR promoters, and are terminated by tL1 and tP1 respectively
how does transcription take place beyone the tL1 and tP1 terminators
the N protein is an anti-terminator and alters the RNAP in Nut sites
what is transcribed by PL and PR in the early stage
PL transcribes CIII, xis and int and PR transcribes CII, O, P and Q
what is the role of the Q protein
it modifies RNAP at the Qut sites to allow transcription of the head and tail regions of the genome in the late stage
what is the role of Cro in the late stage
it prevents the synthesis of CI (a repressor) and further transcription of early stage genes
which new promoters are available to bind RNAP in the lysogenic pathway
Pint for the transcription of the int gene and PRE for the transcription of CI
which genes are required for integration and excision
int is needed for integration and both int and xis are needed for excision
what is the role of CII and when is it most abundant
it stimulates the production of int. it is protected from degradation by CIII and is most abundant when the host proteases are low which is during periods of starvation
how is int regulated in the lytic phase
both int and xis are transcribed, but so is sib which degrades int mRNA so levels of int are lower relative to xis
what promotes the CI gene for the lambda repressor
it is initially transcribed by the PRE promoter, then later by the PRM promoter
what differences are there in the affinity of the lambda repressor and Cro proteins
lambda repressor has high affinity for OR1, and as a result of being a dimer OR2, whereas Cro proteins have a higher affinity for OR3
how does the lambda repressor positively and negatively regulate simultaniously
by binding to OR1/2 it stimulates PRM and therefore increased the amount of transcription of CI to produce more of the lambda repressor (+). this simultaneously blocks the PR promoter which is needed for the transcription of the Cro and other lytic genes (-)
what is the result of lambda repressor self-regulation
it allows the repressor to be made in excess which is used to prevent super infection
describe the structure of the lambda repressor
each monomer has two domains, an amino and a carboxyl, but exists as a dimer
what are the roles of the two domains of the lambda repressor
the C domain is used to bind to another monomer to form a dimer. the N domain is what binds to the DNA in the OR operon
when will Cro proteins bind each of the OR operon domains
it will initially bind to OR3 as it has highest affinity for this. it will only bind OR1/2 when it is at a very high concentration as this blocks the transcription of more Cro genes
how do the lambda repressor and Cro proteins bind to the DNA of the OR operon
the DNA domains have two- fold mirror symmetry and two major grooves- one for each monomer of the dimer to bind
what do both the lambda repressor and Cro proteins use to bind to the OR DNA
they both use a helix- turn- helix motif
what determines the affinity of Cro and lambda repressor proteins for the OR DNA
differences in the amino acid- base pain interactions across the three OR domains
what is a lysogen
when the lambda repressor genome has been integrated into the bacterial chromosome
what is the significance of the CI gene in regards to inducing the lysogen
the CI gene is the only one transcribed in the late stages of the lysogenic pathway. it is the deactivation of CI that induces the lysogen to enter the lytic pathway
what is RecA
a host DNA repair protein which is activated by UV light
how does RecA deactivate CI
it cleaves the bond between the C and N domains of the lambda repressor so they can’t dimerise and lose affinity for the OR operon domains. PRM isn’t stimulated and PR isn’t blocked
what is the normal function of RecA and how is it exploited by the lambda phage
it is normally part of the bacteria’s SOS response to cleave bacterial protein LexA. the phage exploits it due to the structural similarity of LexA and the lambda repressor
how does excision happen in the induction of the lysogen
due to the transcription of equal amounts of the xis and int genes
why is sib not transcribed in the induction of the lysogen
it is only brought close to the int/xis genes when the genome circularises upon entering the lytic cycle
