L13 - regulation of prokaryote gene expression Flashcards
define constitutive expression and give examples
genes transcribed continuously
genes for routine tasks eg RNA pol, tRNA
(housekeeping genes)
what is meant by regulated expression
genes whose products are only needed in response to changes in environment and so their expression is regulated
what is meant by -ve control of transcription
involves repressor protein
inhibits transcription
at what stage are most prokaryotic genes regulated?
transcriptional regulation
what is meant by +ve control of transcription
involves activator protein
promotes transcription
what part of gene do activators/ repressors bind to
regulatory sequence
define operon
clusters of genes involved in same functions undergoing coordinated Tscription controlled by one shared regulatory region
are operons found in eukaryotes
no
what is the lac operon responsible for
producing 3 polypeptides needed to use lactose as an energy source
describe structure of lac operon left to right
laci promoter – laci– regulatory/promoter region (CAP binding site, promoter sequence, operator lacO) –lacZ – lacY – lacA
what is found in the regulatory/promoter region of the lac operon
CAP binding site
promoter
operator
what is laci
region that codes for repressor protein
what does lacZ encode and what is its function
B galactosidase
breaks lactose into glucose+galactose
what does lacY encode and what is its function
lactose permease
involved in lactose uptake
what does lacA encode
transacetylase
is laci expressed constitutively or regulated?
constitutively
what part of the lac operon does RNA pol bind to
promoter
what binds to the CAP binding site
CAP-cAMP
what binds to lacO (operator)
lac repressor protein
is laci part of the lac operon? why / why not
no
it has its own promoter
describe structure of lac repressor protein
contains
- DNA binding domain (that binds to lacO)
- allosteric domain (that binds the inducer, allolactose)
what binds to the DNA binding domain of the lac repressor protein
lacO
what binds to the allosteric domain of the lac repressor protein
the inducer - allolactose
what is allolactose?
produced in breakdown of lactose
binds to repressor protein
what happens in absence of lactose
- no lactose = no allolactose
- lac repressor protein binds to lacO (operator)
- transcription prevented
what happens in presence of lactose
- residual B galactosidase produces allolactose from lactose
- allolactose binds to allosteric domain of lac repressor protein and changes its shape
- repressor protein cant now cant bind to lacO
- transcription takes place
why is +ve regulation of the lac operon needed
the inducer repressor complex isnt sufficient for lactose metabolism
describe +ve regulation of the lac operon
- in absence of glucose cAMP is produced which binds to CAP
- CAP-cAMP binds to promoter region (CAP binding region)
- stimulates transcription
what is CAP
catabolite activator protein
will expression of the lac operon occur in the presence of glucose?
yes but not much , as when glucose is present there is no cAMP so no +ve regulation of the lac operon
describe the laci- mutation
produces mutant lac repressor protein that cant bind to operator
what happens as a result of a laci- mutation
- lac operon constitutively expressed
- B galactosidase always present
describe lacO- mutation
mutated operator sequence that repressor protein cant bind to
what happens as a result of a lacOc mutation
lac operon constitutively expressed
B galactosidase always present
describe laci ^s mutation
mutant repressor protein which allolactose cant bind to
what happens as a result of a laci ^s mutation
transcription always repressed
what is arabinose
pentose sugar from plant cell walls released when plant ingested so can be used as C source for E.coli in gut
function of araB araA araD
breakdown arabinose until it can enter glycolysis
function of araC
gene for araC protein
is araC transcribed constitutively or regulated
constitutively
what happens in the absence of arabinose
- araC protein binds to araI and araO2 forming a loop 2. loop prevents RNA pol and CAP-cAMP binding to para
- transcription inhibited
what happens in the presence of arabinose
- arabinose binds to araC protein breaking its connection between araI and araO2
- loop opens up to straight line
- second araC - arabinose complex binds to araI which interacts with DNA pol increasing its affinity for para
- promotes transcription
describe +ve regulation of arabinose operon
CAP -cAMP bind to CAP binding site (when glucose absent) which further increases RNA pol affinity for para
how much does arabinose expression increase in the presence of arabinose
300x in 3 sec
