Lecture 16 Flashcards
When might lac utilisation be low despite high lactose levels?
Bacteria will attempt to eat the most useful substrate, this can be done by having the more useful substrate lower the production of the less useful substrates operon. E.g with lac gene glucose presence will make cAMP levels low and this will lower production of lac genes.
What is a virus and bacteriophage?
A virus is an infectious agent that must grow or reproduce inside a host cell, it is an obligate intracellular parasite and is composed of nucleic acid genetic material and a protein coat(sometimes lipids in this). Bacteriophages are viruses which infect bacterial cells.
What is phage lambda and what is its lifecycle?
phage lambda infects E.coli and has double stranded DNA. Its lifecycle is infection followed by other the lytic pathway (many viral chromosomes, viral assembly and cell lysis) or the lysogenic pathway (recombination and integration, turning into a prophage, possible prophage induction (swaps to lytic cycle) or more cell production.)
What is a virulent phage, temperate phage, lysogen and prophage?
a virulent phage is a phage that can only use the lytic cycle, a temperate is a phage that can be lytic or lysogenic, a lysogen is a host cell that is harbouring a prophage and a prophage is a the latent form of a temperate phage.
Which pathway is default and how do both occur?
The lytic pathway is effectively the default pathway unless signals send the bacteriophage down the lysogenic. The genome starts off linear in phage head and following injection the lambda genome circularises aided by cohesive end sites (cos sites). It is organised into modular sites. For the lytic cycle the head and tail proteins are synthesised, DNA is packaged into heads, tails are added and then the host is lysed, releading the new phage. Alternatively the lysogenic cycle involves lambda integration into the genome, stable prophage maintenance and prophage being passed to daughter cells. This can be done via attP(attachment on phage) and attB(attachment on bacteria), this will lead to site-specific recombination, requires Int enzyme (integrase), lambda will be linearised in bacterial chromosome.
What would be some good times to choose a particular lifecycle?
A good time to make more phages is with high growth of bacteria, a good time to integrate is if the cell doesn’t have enough resources to produce many phages, a goot time to pack up and leave might be if the host cell is damaged and likely to die or plenty of resources are available. But there is much more to it than just these factors.
What main gene expression occurs throughout a lambda phage?
Gene expression starts off the same. Following the decision to be lytic or lysogenic the gene expression patterns differ, many more in lytic. Very early expression involves cro(PR replication pathway) (Cro protein, major part of establishing lytic growth, DNA-binding protein, represses transcription and hence promotes lytic) and N(PL)(encodes and anti-terminator protein that enables transcription past the 2 terminators, leading to early gene expression). CII is a DNA-binding protein activates transcription (promotes lysogenic), CI is a DNA-binding protein that can do both.
CII is the deciding protein, host proteases will degrade this, with healthy cells producing high levels meaning in actively growing cells the CII gets decraded and Cro wins, meaning the lytic pathway is taken in healthy cells and vice versa.
Cro represses expression of cI, all early genes then itself and the replication genes.
Q is an anti-terminator that enables expression of late lytic genes (head, tail and lysis genes).
CII turns on int, leading to integration of lambda into host chromosome and the PRE (promoter for repressor establishment) which will turn on CI, this activates its own expression and binds to OL and OR to repress all other phage genes (also maintains the lysogeny by continuing this.
