The control of gene expression enable individual bacteria to adjust their meta- bolism 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 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; con- sists of an operator, pro- moter, and the genes they control

The “switch” for turning the genes off and on is called an operator. It is positioned within the pro- moter or between the pro- moter 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 synthe- sized from E. coli from a pre- cursor 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 metabol- ism begins with the hydroly- sis 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 galactosi- dase 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