Question | Answer |
Structure of the lac repressor | tetramer (dimer of dimers) |
the lac repressor binds | LacO |
how the lac operon moves along DNA | diffusion |
Repressor does not block polymerase binding. Rather, | it inhibits open complex formation and elongation |
Two mutations that allow constitutive lac expression to occur | lacI or Oc mutation |
trans mutation: lac expression | lacI (repressor protein) |
cis mutation: lac operon | mutation in the operator sequence |
lactose & no glucose | expression of lac operon |
lactose & glucose | no expression of lac operon |
Class I CAP-dependent promoter | only require CAP-cAMP; binding site does not overlap promoter |
Class II CAP-dependent promoter | Only require CAP-cAMP; binding site replaces -35 of promoter |
Class III CAP-dependent promoter | Require multiple activators |
When glucose levels are low, cAMP binds to a protein known as | Catabolite gene Activator Protein (CAP) |
CAP is a homodimer of 210 aa subunits that has binding sites for | cAMP and DNA |
When glucose levels are low (cAMP high), CAP-cAMP complex binds near | lac promoter |
binding of CAP-cAMP complex near promoter does what? | stimulates transcription of lactose metabolizing genes |
CAP can not bind DNA in absence of cAMP | cAMP thus allosterically regulates CAP |
CAP responds to | glucose |
Lac repressor responds to | lactose |
restricts the expression of genes required for
catabolism of arabinose, lactose, and other simple sugars in the presence of
glucose, even when this secondary sugar are abundant | Catabolite repression |
Glucose mediated catabolite repression is facilitated by | cAMP |
CAP stimulates promoter recognition of | RNAP |
In absence of lac repressor, expression is still | low |
CAP-cAMP complex activates by two mechanisms: | (i) direct interaction
with RNAP (α subunit) and (ii) Conformational change - DNA bending
(~90 o ) CAP interacts with the RNAP via its C-terminal domain |
AraC absent | araC transcribed + araBAD at basal level |
cAMP and L-arabinose are low | AraC represses araBAD transcription |
cAMP and L-arabinose abundant | araBAD transcription activated |
trp operon | convert chorismate to tryptophan |
trp operon modes of regulation | repression and attenuation |
When this protein's levels are high, trp binds to it | L-tryptophan |
trp regulation is feedback inhibition, aka | negative feedback loop |
Attenuator is an RNA structure located within | 5' leader of nascent transcript |
TrpR acts as a dimer, binds tryptophan, which induces a conformational change that allows | the protein to bind the operator
sequence |