chapter 12 part 2 Flashcards
where does allolactose bind to the lac repressor
allosteric domain
the formation of the inducer-repressor complex alters what?
DNA-binding domain of the repressor and prevents it binding the operator
when allolactose binds to the repressor, what is removed
negative control (repression)
2 requirements for lac activity
- lactose present
- glucose absent
will glucose or lactose be used first?
glucose
positive control mechanism with glucose and lactose
catabolite repression
what do high levels of glucose inhibit?
adenylate cyclase (AC)
what does AC convert ATP to
cAMP
cAMP binds to what?
catabolic repressor protein (CAP)
what does the Crp-cAMP complex bind to
lac promoter
what happens when the Crp-cAMP complex binds to the promoter
- strengthens RNA pol interaction
- physically distorts CAP binding region to expose major grooves
what does bending the DNA at the lac promoter do?
exposes major grooves and allows RNA polymerase to bind efficiently
I-
unable to bind to operator
Is
super repression - unable to bind inducer
Z-
no functional B-galactosidase
Y-
no functional permease
A-
no transacetylase
Oc
fail to bind repressor protein - continuous transcription
P-
fails to bind RNA polymerase
complementation analysis is carried out in _________ _________ produced by conjugation between F’ (lac) and F- bacteria
partial diploids
constitutive mutants
certain mutations where genes are transcribed continuously whether or not lactose is available
lac- cells
unresponsive to the presence of lactose
genetic mapping of constitutive mutants localizes them to the _____ and ______ regions
lacO and lacI
lać operator mutations are exclusively _______-acting
cis
cis-acting
influence transcription of genes on same chromosome
what is cis-acting typical of
DNA sequence mutations that don’t code for proteins
- DNA binding sites
lacI produces a regulatory protein that is ________-acting
trans
trans-acting
capable of diffusing and interacting with both operators in a partial diploid
what is trans-acting typical of
protein mutations
non-inducible operon
Z and Y genes aren’t expressed, even in presence of lactose
what does the non-inducible operon mutation cause
allosteric domain to be altered so that allolactose can’t bind to it
super-repressor mutation
non-inducible - allosteric domain altered so allolactose can’t bind to it
what does the operator overlap with
the promoter
the repressor bending physically interferes with what?
RNA polymerase binding
what does the tetrameric repressor bind to?
O1 and O3
what does binding of the tetrameric repressor to O1 and O3 induce
DNA loop formation that brings O1 and O3 close togehter
the loop structure includes part of the promoter and blocks what?
access by RNA polymerase
Oc mutations
disrupt the two-fold symmetry of the O1 segment
- DNA loop can’t form, RNA pol can bind to promoter
is the tryptophan operon involved in anabolic or catabolic pathways
anabolic
feedback inhibition
activity of the end product blocks transcription of the operon
the tryptophan operon is ___________ and _________
repressible and attenuated
attenutation
can fine-tune transcription to match the immediate needs of the cell
what does the trp operon contain
- five structural genes
- regulatory region with promoter (trpP) and operator (trpO)
- leader region (trpL)
- attenuator region
5 structural genes of trp operon
E, D, C, B, A
the sixth gene outside the trp operon encodes what?
trpR protein - repressor protein that is activated when bound to trp
tryptophan acts as a _________ by binding to the trp repressor and activating it
co-repressor
what happens when trp is absent
repressor protein can’t bind to operator allow for transcription of operon
when does the trp operator turn on
when there is no tryptophan present - makes trp
what happens when trp is present
activated repressor binds to trp O and prevents transcription of the operon
- turns off production of trp
true or false: trpR- cells are produce trp constitutively
false - trpR- strains have higher rates of operon transcription in the presence of trp than trpR+ strains, but not 100%
what is attenuation controlled by
162-bp trpL region
what does the trpL region contain
4 repeat DNA sequences
- mRNA produced contains complementary repeats and start/stop codons for a 14 amino acid polypeptide
what can the four repeat sequences form
different stem-loop structures
among the codons for the short polypeptide of trpL are 2 what?
back-to back codons for tryptophan
- sensors for availability of trp in cell
what are the sensors for the availability of trp in the cell
2 back-to-back codons
what 2 stem-loop structures are central to attenuation?
3-4 and 2-3
- 1-2 plays minor role
3-4 stem loop of mRNA
- termination stem loop - halts RNA polymerase progress along leader region of DNA
what is region 4 followed by
poly-uracil sequence that function similarly to intrinsic termination of transcription by bacteria
what may the 3-4 stem loop be preceded by?
1-2 stem loop
2-3 stem loop of mRNA
antitermination stem loop - forms when region 1 is not available for pairing and thus prevents region 3 from interacting with region 4
- allows RNA polymerase to continue through leader sequence
transcription along regions 1 and 2 allows for what to form?
1-2 stem loop, and slight pause in transcription
what does the slight pause that comes from transcription of regions 1 and 2 allow?
ribosome binds to start codon in trpL (translation begins)
with adequate supply of trp, codons 10 and 11 are easily translated which leads to what?
ribosome moves to the stop codon, partially obscuring regions 1 and 2
what happens as a result of the ribosome moving to the stop codon
allows regions 3 and 4 to pair, which causes termination of transcription by attenuation
without an adequate supply of trp, where does translation stall
codons 10 and 11
what happens when the ribosome stalls at codons 10 and 11
ribosome obscures region 1 and allows regions 2 and 3 to pair
what happens when regions 2 and 3 pair
creates anti termination conformation, allowing for continued transcription of genes needed for production of trp
mutating of one codon 10 and 11 results in what
alters attenuator responsiveness to trp
mutating both codon 10 and 11 results in what
abolishes attenuator’s ability to sense trp
mutation of regions 3 and 4 results in what
prevents stable binding between them
attenuation also represses transcription where?
in several amino acid operon systems such as E. coli and Salmonella
