12.3: Gene Regulation in Prokaryotes- the Tryptophan (trp) Operon

Let’s look at two ways that E. coli controls cellular levels of metabolites such as amino acids. Recall feedback inhibition, in which the synthesis of a substance is inhibited when an excess of the substance binds to and allosterically inhibits the activity of an enzyme in the pathway to its own synthesis (Figure 5.4). Synthesis of the amino acid tryptophan (trp) is allosterically regulated in this way. The 5 polypeptides encoded by the trp operon comprise the three enzymes in the trp biosynthetic pathway as shown in Figure 12.10: holoenzyme 4/5 (Enzyme 1), protein 3 (Enzyme 2) and holoenzyme 1/4 (Enzyme 3). When cell growth slows down, the cells initially accumulate excess trp, some of which will bind to and allosterically inhibit the dimeric Enzyme 1, the first enzyme in the pathway. The result of the inhibition of intermediate B production is, of course, a slowdown in trp synthesis.

Bacterial cells use a second feedback mechanism to slow down trp synthesis. During rapid growth, the trp operon is active and will continually synthesize the amino acid. The “on” state of the trp operon is illustrated in Figure 12.10.

Figure 12.10: Transcription of the five-gene tryptophan (trp) operon: the five resulting proteins are enzymes in the pathway for trp synthesis. A trp-repressor gene to the left of the operator is always transcribed.

On the other hand, if tryptophan consumption slows down, tryptophan accumulates in the cytoplasm. Excess tryptophan will then bind to the trp repressor, a protein encoded by the repressor protein gene. The trp-bound repressor then binds to the trp operator, blocking RNA polymerase from transcribing the operon. The repression of the trp operon by trp is shown in Figure 12.11 (below). In this scenario, tryptophan is a co-repressor. The function of a corepressor is to bind to a repressor protein and change its conformation so that it can bind to the operator.

Figure 12.11: When there is sufficient tryptophan (trp) in the cells, it acts as a corepressor, excess trp binds to and changes the conformation of the trp-repressor protein, which then recognizes and binds the trpoperon operator (O), blocking RNA polymerase from moving past the promoter (P), thus repressing operon transcription.

219 Repression of the Tryptophan (trp) Operon