In the TRP operon, what occurs when tryptophan levels are low?

When tryptophan levels are low, the operon is activated, allowing for the transcription of genes necessary for tryptophan synthesis. In the case of the TRP operon, this activation is a response to the need for more tryptophan in the cell. The operon consists of several genes that code for enzymes involved in the biosynthesis of tryptophan. When tryptophan concentrations drop, the cell must produce more, and this is accomplished by the binding of RNA polymerase to the promoter region, allowing for the transcription of these genes.

The regulation mechanism operates by the absence of tryptophan preventing the formation of a repressor-tryptophan complex, which would otherwise inhibit the transcription. Therefore, when tryptophan is scarce, the repressor is inactive, leading to the activation of the operon and subsequent gene expression to produce more tryptophan.