BIO230 LECTURE 3
Sept 17, 2013
The notes only make sense when read alongside the slides.
If something's on the slides, it probably isn't in my notes.
Prokaryotic Gene Regulation:
DNA cis elements: regulatory elements (2 kinds)
one type of gene regulatory protein turns repressors on
the other type turns repressors off
The only time you turn the lactose on is if you are all out of
because glucose is the preferred energy source for
competition between RNA polymerase and repressor
protein for promoter binder
activator protein recruits RNA polymerase to the promoter
to activate transcription
Regulatory elements can also be found:
-far upstream of gene
-downstream of gene (eukaryotes) -within gene (introns, eukaryotes)
Lac repressor is a tetramer (four identical subunits) and
can bind two operators simultaneously
Gene regulatory switch that controls lifestyle of BL
(molecule that infects bacteria)
lands on a bacterial cell and injects its DNA into bacterial
Two proteins repress each others synthesis (a simple
transitional circuit) one protein affects expression of other
Bacteriophage Lambda Lifestyle:
Prophage pathway: minimal amount of BL expression
But if cell is compromised, BL chooses lifestyle of Lytic
Pathway. Results in a lot of BL expression.
Original cell undergoes lysis and then tonnes of BL bacteria
released into environment
2 regulatory proteins involved from switching between
Prophage pathway and Lytic Pathway. they are:
1. lambda repressor (cI)
2. cro protein two proteins repress each other's synthesis
In prophage stage, lambda repressor protein active.
inhibiting expression of cro. positive regulator of self.
negative regulator of cro.
repressor protein one of the only proteins being
in lytic stage, cro protein takes over. cro expressed. cro
turns off lambda repressor protein. cro is positive
regulator of self. cro negative regulator of lambda
State 1 of Prophage
lambda repressor blocks synthesis of cro
activates its own synthesis
most bacteriophage DNA is not transcribed
State 2 of Lytic
Cro occupies operator.
blocks synthesis of repressor protein
allows its own synthesis
most bacteriophage DNA is extensively transcribed
What triggers the switch? To go from Prophage to Lytic. under favourable environments, will have Prophage.
Under harsh environments, bacteria will undergo lytic.
often under harsh environments DNA of bacteria
damaged. Dna repair proteins deployed in bacteria to
DNA repair proteins actually inactivate lamdba repressor
causing lytic state.
when lambda repressor inactivated, lambda repressor no
longer supressing cro. cro will then express causing lytic
host response to DNA damage inactivates repressor
-switch to lytic state
under good growth conditions, repressor protein turns off
cro and activates itself. positive feedback loop.
-maintains prophage state
example of a transcriptional circuit
Cro-repressor switch is a flip-flop device
inhibit each other.
when one is active, it inhibits the other.
either one or the other being expressed feed forward loop:
both A and B required for the expression of Z
Positive feedback loop of A
A sends transient signal which expresses protein A
created protein A then comes back and turns on another
will end up with a lot of