The first well studied mechanisms for the regulation of gene expression in prokaryotes involved the control mechanisms for the production of enzymes involved in utilization of lactose by Escherichia coli. The lactose operon responds to increasing lactose levels (lactose is the inducer) to increase production of β-galactosidase, β-galactosidase permease, and thiogalactoside transacetylate by triggering the transcription of mRNA.

Many bacterial genes are clustered in operons, under the regulatory control of repressor and stimulatory proteins. An operon is a coordinated unit of gene expression consisting of a regulator gene, an operator site, and a set of structural genes. The regulator gene codes for a repressor molecule, which can travel and act as an 'off' signal on the operator gene. Operons also contain the promoter site, partially overlapping the operator, which is the RNA polymerize binding site. Inducer binds the repressor and prevents its interaction with the operator.

β-galactosidase is an enzyme catalyzing the hydrolysis of lactose into glucose and galactose in Escherichia coli. The production of β-galactosidase is controlled by regulation of a well-studied inducible operon, i.e. the production of the enzyme is induced by the presence of lactose.

Because it is more advantageous to use all available glucose before beginning lactose breakdown, the lac operon is also subject to catabolite repression. RNA polymerase binds to the promoter site much better if CAP, catabolite gene activator protein, is bound to CBS (CAP binding site), which occurs when cAMP is high (glucose low). In other words, the lac operon is under dual control, not only induction by lactose but repression by glucose. Catabolite repression acts through regulation by CAP protein containing bound cyclic AMP. Glucose and cyclic AMP concentrations are in rough inverse proportion. Cyclic AMP, a widely seen hunger signal, stimulates transcription of many inducible operons. While in mammalian cells, cyclic AMP triggers protein kinases, cAMP in bacteria acts at the level of gene expression. cAMP binds to CAP (catabolite gene activator protein). CAP creates an additional interaction site for RNA polymerase.

trp operon, blocked by repressor containing bound tryptophan, is another well-studied model of the regulation of gene expression in bacteria. Complex of tryptophan and repressor binds tightly to the operator. Tryptophan induces a conformational change leading to the formation of a DNA-binding surface. The trp operon also contains an attenuator site, which is a controlled termination site. More RNA polymerase gets past the attenuator site as tryptophan levels decrease.

This is pretty important material for the MCAT. Most biology textbooks have excellent figures on the lac and trp operons which you should study until the above discussion can be read and comprehended with ease.