Compare and contrast the lac and trp operons in E. coli

The lac operon repressor works by preventing transcription of the B Gal gene. The trp operon repressor works by attenuating mRNA at different sites based on repressor presence or absence.

Lactose  ----------------------------------->  Glucose + Galactose

                ß-galactosidase

Several proteins involved in lactose metabolism in the E. coli cell. They are:

ß-galactosidase - converts lactose into glucose and galactose
ß-galactoside permease - transports lactose into the cell
ß-galactoside transacetylase - function unknown

All of the genes involved in controlling this pathway are located next to each other on the E. coli chromosome. Together they form an operon. The following is the genetic structure of the operon.

Lactose is not the preferred carbohydrate source for E. coli. If lactose and glucose are present, the cell will use all of the glucose before the lac operon is turned on. This type of control is termed catabolite repression. To prevent lactose metabolism, a second level of control of gene expression exists. The promoter of the lac operon has two binding sites. One site is the location where RNA polymerase binds. The second location is the binding site for a complex between the catabolite activator protein (CAP) and cyclic AMP (cAMP). The binding of the CAP-cAMP complex to the promoter site is required for transcription of the lac operon. The presence of this complex is closely associated with the presence of glucose in the cell. As the concentration of glucose increases the amount of cAMP decreases. As the cAMP decreases, the amount of complex decreases. This decrease in the complex inactivates the promoter, and the lac operon is turned off. Because the CAP-cAMP complex is needed for transcription, the complex exerts a positive control over the expression of the lac operon.

For the trp operon

My advanced genetics class worked together to answer your question.

Lac is catabolic and trp is anabolic. A catabolic reaction breaks down its substrate, and an
anabolic reaction builds a product. Both operons have a general structure of a regulatory gene, the
promoter, an operator, and then the genes. They both use repressor proteins that bind to the operator,
and RNA polymerase which binds to the promoter. The Lac operon has 3 operators, and the trp operon
has 5 operators. The lac repressor binds allolactose, and the trp repressor binds tryptophan. Allolactose
induces transcription and removes the repressor from the operon, and tryptophan is a corepressor that
binds to the repressor to inhibit transcription. cAMP induces transcription in the lac operon by binding
to the CAP protein (which binds to the CAP site). The CAP protein activates transcription. The trp
operon has a leader sequence which is used for attenuation. The leader sequence measures how much
trp is available because it contains two trp codons.