What occurs when high levels of tryptophan are present?

When high levels of tryptophan are present, transcription is blocked by the repressor protein. In prokaryotes, particularly in the tryptophan operon (trp operon), tryptophan acts as a corepressor. When tryptophan levels are high, it binds to the repressor protein, leading to a conformational change that allows the repressor to attach to the operator region of the operon. This binding prevents RNA polymerase from transcribing the genes necessary for tryptophan synthesis, effectively shutting down the production of more tryptophan and ensuring that the cell conserves resources.

This regulatory mechanism is an example of feedback inhibition, which helps to maintain homeostasis within the cell. Since the pathway to produce tryptophan is effectively turned off when it is abundant, cellular energy and resources are not wasted on processes that produce something the cell does not need at that moment.