Download Gene Regulation

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We have discussed the concept that all genes
for an organism are found in all cells that
contain a nucleus
◦ But, only the proteins for that cell are “turned on”
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Would the proteins for a given cell always be
“required” though?
Cells can respond to environmental cues to
regulate which proteins are actually needed
We have seen this in people living in high
altitudes
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In addition to the promoting regions found
on DNA, there is also a switch segment
◦ This is called an operator
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It is located between the promoting region
(recall the TATA box) and the genes to be
expressed
Collectively, all three portions are called an
operon
Specific proteins, called repressors, fit into
the operator
When there, they block the RNA polymerase
from binding
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But, if it is almost always present, how do the
genes ever get expressed?
The repressor is what we call an allosteric
molecule – it has two different shapes
The change is controlled by other molecules
that can bind to it
This controls whether the repressor can bind
or not
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This common example is found in E. coli
Lactose can be broken down and used by
these bacteria by the use of an enzyme called
β-galactosidase
◦ Found on a lactose operon, abbreviated lac operon
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Why make the enzyme if no lactose is
present?
Lactose acts as an inducer
◦ It inactivates the repressor
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So, the presence of the sugar induces the
expression of the enzyme that metabolizes it
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Also on the operon is a gene for a membrane
protein that facilitates to entrance of lactose
into the cell
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http://www.youtube.com/watch?v=oBwtxdI1
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It is also possible to have a molecule that
activates the repressor
E. coli is also capable of synthesizing its own
tryptophan via enzymes
These enzymes are found on the trp operon
Unlike before, the presence of tryptophan
changes its shape so it fits
So, the cell is not going to make tryptophan
when in can just absorb it