control of gene expression Flashcards
(34 cards)
why study E.coli
bacteria are key human pathogens
bacterial gene expression machinery is the target for some antibiotics
E.coli is an important host for production of recombinant proteins for research and industrial/medical purposes
provides a framework for more complex organisms
transcription in E.coli
- promotor
immediately upstream of transcribed region
start signal - transcribed region
one mrna encodes more than one protein to allow co ordinated expression of a group of genes - terminator = stop signal
polycistronic
one mrna encodes more than one protein
E.coli sigma 70 promotor
binding site for rna olymerase
40-60 bp region upstream of txn start site
promotors
dictate where txn begins
strength of promotor dictates how efficiently transcription is initiated
strength dictated by sequence
E.coli RNA polymerase
Magnesium dependent
multisubunit
core and sigma factor = holoenzyme
holoenzyme
compound of enzyme and coenzyme
Core
catalyses transcription
cant recognise/bind to promotor
sigma factor
binds to core to convert it to holoenzyme
directs recognition of promotor sequences
sigma 70
primary sigma factor which directs initiation from promotors of most genes in growing cells
alternative sigma factors
utilized under different environmental conditions
they recognise promotors of genes appropriate to environmental conditions
factor independent termination
series of 4-10 AT base pairs
a G+C region with a palindromic sequence that immediately precedes the series of A-T base pairs
Rho dependent termination
rho factor = six identical subunits
rho is a helicase that unwinds RNA-DNA and RNA-RNA duplexes
powered by ATP hydrolysis
consensus sequence
calculated order of most frequent nucleotides
transcription initiation regulated by
negetive regulatory factor called repressors
positive acting factors called activators
when does high transcription of lac structural genes occur
lactose present
glucose is absent
lac repressor
key to regulation of lac operon in response to lactose product of lacl gene is a repressor 360 amino acid protein forms homotetrameter binds to lac operon
lactose effect on the lac operon
lac operon induced when lactose provided
inducer is allolactose
allolactose and the lac repressor
causes conformational change
binding subunits separate by 3.5A
affinity for operator reduced by 1000
how does glucose influence transcription of lac operon
glucose levels influence cAMP levels cAMP made from ATP by adenylate cyclase glucose travel into cell inhibits adenylate cyclase and prevents cAMP accumulation as glucose levels drop cAMP accumulates cAMP binds to and activates CAP
glucose present
no lactose
(lac Operon)
no residual txn
repressor blocks txn
CAP doesnt bind
no activation
no glucose
high cAMP
no lactose
(lac Operon)
no resifual txn
repressor blocks RNA pol
glucose present
low cAMP
lactose present
(lac Operon)
little txn
CAP doesnt activate
no glucose
high cAMP
lactose present
high txn
no repressor
CAP activates
