23 the lac operon Flashcards
the activator or repressor modulates what?
the binding of RNAP to the gene promoter
what signal causes the repressor to detach from the operator site?
allolactose
what signal causes the activator to bind to the activator site?
cAMP
how is lactose metabolised in E. coli
mostly hydrolysis by beta-galactosidase to galactose and glucose, and a tiny bit to allolactose
what is the lac operon?
a gene controlled by a single promoter with additional regulatory sequences (O and CRP) and with a single coding sequence for 3 separate proteins
what is the CRP site?
binding site for activator causing positive regulation
what is the I site
is it the repressor gene => not part of the operon
what is the O site?
the operator => binding site for the repressor, negative regulation
what are the 3 proteins for lactose metabolism:
beta galactosidase, lactose permease and thiogalactoside transacetylase
how does differential expression of the three proteins work?
translational control
what happens when there is no lactose?
repessor expressed constititively. repressor is attached to the operator, RNAP cannot bind the promoter
what happens when lactose is present?
cell metabolises it to glucose, galactose and allolactose. allolactose is the molecular signal. binds to the repressor causing conformational change, allows repressor to detach. RNAP is free to bind promoter, synthesize mRNA
how does positive regulation happen?
cell produces cAMP, cAMP binds to CRP protein => CRP changes conformation, binds to CRP site, helps RNAP bind to promoter and has max expression of lac proteins
what’s the structure of the repressor?
4x monomer proteins, each with one DNA binding domain. 3 operator regions: repressor either occupies O1-O2 or O1-O3. RNAP is physically blocked
what is the lac repressor dna binding motif
an alpha helix -> at the operator sequence the alpha helix binds to bases in the major groove and the backbone
