Download Figure 18.19 Regulation of a metabolic pathway

The control of gene
expression enable individual
bacteria to adjust their
metabolism to environmental
change
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 trp operon: regulated synthesis of repressible
enzymes
The trp operon: regulated synthesis of
repressible enzymes
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 trp operon: regulated synthesis of repressible
enzymes
The repressor is a product
of a regulatory gene. These
are transcribed
continuously, although at a
low rate.
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
The lac operon: regulated synthesis of
inducible enzymes
The lac operon: regulated synthesis of
inducible enzymes
In contrast, an inducible
operon, like the lac operon,
is stimulated (i.e. induced)
when a specific small
molecule interacts with a
regulatory protein.
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 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
The enzyme that catalyzes
this reaction is called
beta galactosidase. In the
presence of lactose it can
increase 1000x in 15 min.
Positive control: cAMP receptor protein
CAP
CAP
Positive control: cAMP receptor protein
CAP
CAP