Full expression of the lac genes requires a transcriptional

1
Full expression of the lac genes requires a
transcriptional activator. The catabolite
activator protein (CAP) binds cyclic AMP and
activates transcription of the lac genes.
lac I P O
lacZYA region
CAP binding site
2
The CAP-cAMP complex binds and assists binding
of RNA pol to the weak lac promoter.
lac I P O
lacZYA region
CAP site
3
Glucose decreases the intracellular level of
cAMP. If glucose is present, the lac genes are
not expressed. Expression of the lac genes
requires (1) the presence of lactose, (2) the
presence of cAMP (i. e. the absence of glucose).
4
CAP also activates transcription of operons for
other catabolic enzymes. The promoters have
imperfect -35 and -10 regions. RNA polymerase
requires assistance from the CAP transcriptional
activator to transcribe these operons.
5
Attenuation
is another method for controlling genes involved
in amino acid biosynthesis.
In the trp operon, the structural genes are
separated from the promoter by a 160 bp leader
region.
P O
trpEDCBA
Leader region
160 bp
6
The leader RNA can form different stem-loop
structures
2
3
1
4
or
The 3 - 4 stem loop is a strong
transcription terminator.
1
2
3
4
UUUUU
7
The leader also contains a small gene for a 14
AA -residue peptide.
5
This gene contains 2 TRP codons.
8
If TRP is low, TRP- tRNA is low. The ribosome
stalls at the UGG codons.
The 2-3 hairpin forms.
2
1
3
4
Transcription continues
9
If TRP-tRNA is available, translation proceeds so
that the ribosome covers sequence 2.
The 3 - 4 hairpin forms and terminates transcripti
on.
1
3
2
4
UUUUU
10
Intermediate levels of TRP stop some of the RNA
polymerase molecules from transcribing the trp
operon. Expression of the genes is turned down
(attenuated).
11
Control of gene expression in pathogenic
bacteria Flagellar phase variation in
Salmonella Salmonella typhimurium makes two
types of flagellin, H1 and H2. This enables the
organism to avoid the host antibody response.
12
The promoter for H2 flagellin gene is on a 993 bp
invertible sequence of DNA.
P
H1 flagellin gene
H2 flagellin
Repressor protein
In one orientation, H2 flagellin and a
repressor of the H1 gene are synthesised.
13
The ends of the 933 bp sequence are flanked by 13
bp inverted repeats.
P
Occasionally, site-specific recombination occurs
between these repeats.
P
14
This results in inversion of the promoter.
P
H1 flagellin
H2 and repressor are not synthesised. H1
flagellin is made.