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”

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

In addition to the promoting regions found on DNA, there is also a switch segment◦ This is called an operator

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

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

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 Why make the enzyme if no lactose is

present? Lactose acts as an inducer

◦ It inactivates the repressor So, the presence of the sugar induces the

expression of the enzyme that metabolizes it

Also on the operon is a gene for a membrane protein that facilitates to entrance of lactose into the cell

http://www.youtube.com/watch?v=oBwtxdI1zvk

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