chapter 12 part 2 Flashcards
what are the two requirements for activity?
lactose must be present, glucose must be absent
what happens when glucose is high?
very little transcription of lac operon
what happens when glucose is low?
transcription is enhances. lots of transcription
what happens when glucose is absent?
catabolite repression (positive control mechanism)
-glucose will be used first even if lactose is present
what do high levels of glucose inhibit?
adenylate cyclase (AC)
what does AC convert to?
cAMP
what does cAMP bind to?
catabolic repressor protein (crp)
-this creates the Crp-cAMP complex which binds to the Lac Promoter
what is crp also known as?
CAP
what is crp transcribed from?
the crp gene
what happens with +lactose/-glucose relating to CAP and cAMP?
CAP-cAMP bidning to the Lac promoter strengthens RNA polymerases interaction with the Lac promoter
-CAP-cAMP binding physically distorts the CAP binding region located immediately upstream the Lac Promoter
-bending of the DNA exposes the major grooves in the Lac promoter allowing RNA pol to bind efficiently
-leads to high levels of transcription in lac operon
I-
repressor is unable to bind to operator
Z-
no function beta galactosidase
I^s
so called super repressor. unable to bind the inducer (allolactose), blocking transcription
Y-
no functional permease
A-
no transacetylase
O^c
fails to bind repressor protein, resulting in continuous (constitutive) transcription
P-
fails to bind RNA polymerase or does so weakly
what is complementation analysis carried out in?
partial diploids produced by conjugation between F’ (lac) & F- bacteria
what is the F’ copy of the operon unable to produce and what about the other copy?
a functional permease (lacY-) & the other copy is unable to prouduce functional beta-galactosidase (Lacz-)
-in combination, the mutations carried by each copy of the operon complement because the wild-type allele of each gene is dominant to the mutant allele
what are constitutive mutants
where genes are transcribed continuously whether or not lactose is available
what do other mutants (besides constitutive) do?
cause cells to be unresponsive to the presence of lactose, & therefore lac-
are lac operon mutations cis or trans
cis-acting- they influence transcription of genes only on the same chromosome
is the lac+ that produces a regulatory protein cis or trans acting?
trans-acting
-is capable of diffusing & interacting with both operators in a partial diploid
non-inducible operon
the Z & Y genes are not expressed, even in the presence of lactose
-causes the allosteric domain to be alterd so that allolactose cannot bind to it; the I^s (for the super-repressor) mutation is dominant to the
-cells are unresponsive to the presence of lactose
what do O^c mutations do?
disrupt the two-fold symmetry of the O1 segment
-the DNA loop cannot form, & RNA polymerase can bind the promoter & initiate transcription
operator & repressor interactions
the operator overlaps w the promoter, allowing repressor binding to physically interfere with RNA polymerase binding
-tetrameric reporessor bind to O1 & O3 & induced DNA loop formation that bring O1 & O3 close together
what does the loop structure include?
part of the promoter & blocks access by RNA polymerase
the tryptophan operon is ____________ and ____________ andd it is responsible for what?
is repressible & attenuated
-the production of tryptophan
does not exactly say the work blanks in the slide
is tryptophan operon catabolic or anabolic?
anabolic - operate through activity of end product to block transcription of the operon - feedback inhibition
-repressible operons
what do repressible operons have as a second regulatory capability?
attenuation- which can find-tune transcription to match the immediate needs of the cell
what does tryptophan act as?
a co-repressor by binding the trp repressor & activating it
what happens when trp is absent
the repressor cant bind to operator, allowing for transcription of the operon
-turning ON the production of tryptophan
how many structural genes does the trp operon contain?
5
what are the protein products of the five structural genes in the trp operon?
trpE, trpD, trpC, trpB, & trpA
what does the regulatory region of trp contain?
a promoter (trpP), operator (trpO), leader region (trpL) that contains the attenuator region
what contains the attenuator region?
leader region (trpL)
what does the sixth region (outside the operon) encode for?
the trpR protein, a repressor protein that is activated when bound to trp
when the is trpR region activated?
when bound to trp
what happens when trp is present?
activated repressor binds to trpO & prevents transcription of the operon
-turning OFF the production of tryptophan
what is the second mechanism for controlling trp operon expression?
attenuation- controlled by the 162 bp trpL region
how many dna repeat sequences trpL region contain?
4
what can the four repear sequences of trpL region form?
different stem loop structures (1,2,3,4)
what are the back to back codons among the codons for the short polypeptide called?
“sensors” for the availability of trp in the cell
what are the two stem-loop structures that are central to attenuation?
3-4 stem loop
2-3 stem loop
about the 3-4 stem loop
termination stem loop
halts RNA polymerase progress along the DNA within the leader regions
what may the 3-4 stem loop be preceded by?
formation of a 1-2 stem loop, which can induced a pause in attenuation
about the 2-3 stem loop of mRNA
antitermination stem loop
-forms when region 1 is not available for pairing & thus prevents region 3 from interacting with region 4
-this allows RNA pol to continue to though the leader sequence & into the structural genes
what happens when there is tryptophan in abundance
transcription across regions 1 & 2 allows for the 1-2 stem loop to form & a slight pause in transcription is just long enough for a ribosome to bind to the start codon trpL (translation begins)
what does transcription across regions 1 & 2 allow for?
the 1-2 stem loop to form & a slight pause in transcription & a ribosome binds to that start codon in trpL
about codons 10 & 11
they are trp codons
-have an adequate supply of trp
-easily translated
-ribosome moves to the stop codon, partially obscuring regions 1 & 2
-allows only regions 3 & 4 to pair- causing termination of transcription attenuation
what happens without an adequate supply of trp
translation stalls at codons 10 & 11
-ribosome obscured region 1, allowing regions 2 & 3 to pair
-creates antitermination conformation, allowing for continued transcription of genes needed for production of trp
what allows for continued transcription of genes needed for production of trp?
antitermination conformation
what else does attenuation repress
transcription in several amino acid operon systems in bactera
ex: e. coli & salmonella
what can reduce the efficiency of the attenuation mechanism
if codons 10 and 11 (the sensors) are mutated to produce different amino acids
if there are mutations of regions 3 & 4, which prevents stable binding between them
