Download Figure 18.19 Regulation of a metabolic pathway

The control of gene
expression enable individual
bacteria to adjust their metabolism to environmental
change
If a bacterial cell is deprived
of the amino acid, tryptophan
which it needs to survive, it
will activate its own
metabolic pathway to make
its own.
Figure 18.19 Regulation of a metabolic pathway
Operon : consists of a
closely related group of
genes that act together
and code for the enzymes
that control a particular
metabolic pathway; consists of an operator, promoter, and the genes they
control
The “switch” for turning
the genes off and on is
called an operator. It is
positioned within the promoter or between the promoter and enzyme-encoding
genes, controlling access to
the genes.
What determines if the
operator is on?
By itself, it is on – RNA
polymerase can bind to
the promoter. It can be
switched off by a protein
called a repressor.
The repressor is a product
of a regulatory gene. These
are transcribed
continuously, although at a
low rate.
Figure 18.20a The trp operon: regulated synthesis of repressible enzymes
Figure 18.20b The trp operon: regulated synthesis of repressible enzymes (Layer 1)
Figure 18.20b The trp operon: regulated synthesis of repressible enzymes (Layer 2)
Tryptophan (trp) is synthesized from E. coli from a precursor molecule in a series
of steps. The trp operon is
said to be a repressible operon
because transcription
of it is inhibited when a specific
small molecule (tryptophan)
binds to a regulatory protein
In contrast, an inducible
operon, like the lac operon,
is stimulated (i.e. induced)
when a specific small
molecule interacts with a
regulatory protein.
Figure 18.21a The lac operon: regulated synthesis of inducible enzymes
Figure 18.21b The lac operon: regulated synthesis of inducible enzymes
The disaccharide, lactose,
is available to E. coli if the
human host drinks milk.
The bacteria can absorb the
lactose and break it down
for energy. Lactose metabolism begins with the hydrolysis of lactose into its two
monosaccharides
The enzyme that catalyzes
this reaction is called
beta galactosidase. In the
presence of lactose it can
increase 1000x in 15 min.
The gene for beta galactosidase is part of an operon,
the lac operon, that includes
two other genes coding for
proteins that function in
lactose metabolism
Figure 18.22a Positive control: cAMP receptor protein
Figure 18.22b Positive control: cAMP receptor protein