Chapter 24 - The Operon Flashcards
Genes with related function are clustered together under the control of a single
promoter
multiple genes transcribed on a single mRNA are called
polycistronic
operon
Group of bacterial structural genes transcribed together
Also contains a promoter and an operator
structural genes
Multiple genes transcribed into one mRNA
Under control of one promoter
regulator gene
Under control of separate promoter
Transcribed and translated into regulator protein
Influences transcription of structural genes
in a negative operon, the regulator protein is a
repressor
in a negative operon the Trans-acting repressor binds to the cis-acting operator and
reduces the rate of transcription
in a positive operon the regulator protein is
an activator
in a positive operon the Trans-acting activator binds to a cis-acting element to …
activate transcription
inducible operons
Transcription is off by default and must be turned on (induced)
repressible operons
Transcription is on by default and must be turned off (repressed)
four types of operons
Negative inducible
Negative repressible
Positive inducible
Positive repressible
what kind of operon is the lac operon
negative inducible
what is the inducer in the lac operon
allolactose
lacI
repressor
lacZ
Beta-galactosidase
lacA
transacytylase
lacY
permease
Properties of lacI transcription
Unregulated gene with a low level of constitutive expression
Transcribed into a mRNA with almost no 5’-UTR
lacOc
constitutive cis-acting mutation
lacI-
constitutive trans-acting mutation
lacIs
uninducible trans-acting mutation
lacI-d
is a dominant negative trans-mutation when found in a heterozygous merozygote
One mutant subunit in the tetramer will block repressor action
lac repressor
The repressor binds to an inverted repeat in the operator
Binds more tightly to the upstream side of the repeat
Within the major groove of DNA
Inducer binding changes the structure of the repressor core so that it can no longer bind into the minor groove
three lac operators
lacO1
lacO2
lacO3
lacO1
is the classic operator at the promoter-lacZ boundary
lacO2
is 410 bp downstream of the transcription start site within lacZ
lacO3
is 88 bp upstream of lacO1 within lacI
the repressor has a higher affinity for which operator
O1
each dimer on the repressor can bind to
a palindromic operator sequence
When the repressor binds to O1-O2 or O1-O3, it induces a
loop into the DNA between the two binding sites
Repressor binding … RNAP binding to the promoter
enhances
Active repressor has … affinity for random DNA
low
Almost all active repressor is bound to either
low-affinity sites in the genome or the operator
Less than what percent of active repressor is free in cell
0.01%
The difference in affinities between the low-affinity and high-affinity sites is large enough that
active repressor is usually bound to the operator
Operator is occupied by repressor …% of the time
96%
what level of transcription allows cell to always be poised for operon induction
basal
When the repressor is inactivated by inducer, affinity of binding to the operator is
reduced
Repressor tetramers are … on DNA
stored
What are the general parameters that influence the ability of a regulator protein to bind to a specific site?
- The size of the genome dilutes the ability of a protein to bind a specific target
- The specificity of a protein counters the effect of genome size
- The amount of protein that is required for effective binding to a specific site increases with increased genome size
- Amount of protein must be in excess of total number of binding sites
