Transcription initiation

1
Transcription initiation RNA pol binds promoter
to form a closed complex.
Unwinding of the region from -10 to the start
point gives an open complex.
2
Abortive initiation occurs -oligonucleotides of
2 to 9 bases are synthesised and released.
3
Finally, s is released, promoter clearance
occurs, and RNA pol core enzyme starts
elongation.
Elongation occurs at 40 to 50 nucleotides per
second.
4
There are 2 types of transcription
terminator (1) Strong (intrinsic,
rho-independent) terminators (2) Weak
(rho-dependent) terminators.
5
Strong terminators consist of a GC rich hairpin
followed by a stretch of 6 U residues.
Hairpin makes RNA pol pause for 60 seconds.
U-A base-pairs are weak - transcript detaches, -
RNA pol dissociates.
6
Weak terminators specify a C rich sequence of 50
to 90 bases.
Rho protein binds to C rich region, migrates
along the transcript and rips it from the
template.
Rho is a hexamer of a 46 kDa polypeptide. It
hydrolyses ATP during termination.
7
Control of Gene Expression in Bacteria Some
genes are expressed constitutively (at the same
level all the time). Promoter strength
determines the level of expression, and the copy
number of the protein in the cell.
8
5 --------------TTGACA----------TATAAT-----------
--3 -35
-10 Strong promoters have ideal -35 and -10
regions. RNA pol binds efficiently. High levels
of mRNA are transcribed. The protein product is
abundant. Weak promoters have imperfect -35 and
-10 regions. Less mRNA is made. The protein is
synthesised at a low level.
9
Induction -synthesis of enzymes in response to
their substrates. In E. coli, lactose induces 3
enzymes b-galactosidase, lactose permease and
transacetylase. These are encoded by the lacZ,
lacY and lacA genes.
10
The lac operon
The Lac I repressor protein binds to an operator
sequence and blocks transcription of the lacZYA
region.
11
P lacI P O lacZ
lacY lacA
lacI mRNA
b-galactosidase
Permease
Acetylase
Lactose neutralises the repressor