 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

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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