Download DNA repair, transposable elements

Announcements
1. Reading Ch. 15: skim btm 425-426
2. Look over problems Ch. 15: 5, 6, 7.
Review of Last Lecture
I. RNA processing in eukaryotes
II. Translation of mRNA into protein - tRNA and ribosomes
III. Three steps of translation
IV. First evidence that proteins are important to heredity
Outline of Lecture 26
Regulating gene expression in prokaryotes
In E. coli, some proteins are found at 5-10 copies/cell;
others are found at 100,00 copies.
How does a cell regulate the levels or amounts of
different proteins at different times - in response to the
environment?
I. Lactose metabolism - the lac operon, an inducible system
II. Trp operon, a repressible system
Background on gene organization
•Operon: a series of gene coding regions (usually
products with related functions) under the control
of a single gene regulatory unit.
•Allows rapid response to changing environmental
conditions.
3 structural genes
lac operon
Bacterial Strategy
• If glucose is present,
– then use glucose as a carbon source.
• If glucose is not present, and if lactose
is present,
– then use lactose (indirectly) as carbon
source.
Levels of enzymes needed to use lactose as carbon source
increase dramatically when lactose is present; enzymes are
inducible and lactose is the inducer.
The -galactosidase Enzyme
lacY gene ->
Permease
(entry of lactose
into cell)
lacZ gene->
lacA ->
transacetylase
lac operon, a polycistronic mRNA
Could the cell make varying amounts of lacZ and lacA RNA?
Genes involved in Lactose Metabolism
Repressor
Regulator
Protein
Enzyme
lactose
?- removes
transporter toxic
byproducts
Normally binds to Operator when lactose is absent,
but when lactose is present, comes off.
The Operon Model - components
pol
R
L
Lac operon when no lactose is present
pol
R
R
pol
pol
R
R
pol
pol
R
R
L
L
pol
pol
R
pol
R
pol
Learning Check
pol
R
L
Will transcription and
translation of Z, Y,and A
enzymes occur?
What would happen if a
wild-type copy of I was
added?
What happens when cell has
both lactose and glucose?
glucose
lactose
b-galactosidase
+
-
-
+
+
-
-
-
-
-
+
+
How does cell prevent breaking lactose down into glucose
and galactose? Catabolite Activating Protein (CAP)
Catabolite Repression of lac Operon
-/+ Glucose
Therefore, for maximal transcription, repressor must be
bound by repressor and CAP must bind CAP-binding site
Goal: efficiency, don’t waste energy
converting lactose, when glucose available
Glucose Inhibits Formation of cAMP from ATP
glucose
lac operon regulatory binding sites
CAP site
Glu
cAMP
lactose
+
-
-
+
-
+
promoter
lac rep
CAP-cAMP
-
+
operator lac coding
lac rep
CAP-cAMP
-
+
+
RNA polymerase
mRNA
Polymerase
binds
here
CAP
DNA
Repressor
3-D structure of
Repressor
bound to
Operator and
CAP bound to
Promoter
Lewis et al. (1996)
Science 271:1247
glucose absent
lactose absent
II. Tryptophan Operon
It makes sense for E. coli to synthesize the
enzymes to make the amino acid tryptophan
ONLY when:
1. The tryptophan concentration is low
AND
2. The tryptophan-charged tRNA is low
Tryptophan Operon - Repressor Binds
when tryptophan is present
mRNA leader sequence involved in
Attenuation - the cast of characters
Translation Event controls
Transcription Event !?!?
High
TryptophantRNA
-->>
Terminates
Transcription
Low
TryptophantRNA
-->>
Transcription
Continues