In negative operon,

Explanation

The tryptophan operon (trp operon) in bacteria is repressible. Here, the repressor is inactive, and it becomes active as DNA DNA-binding protein only when complexed with a corepressor (tryptophan). In the absence of tryptophan, the operator site is open to binding by RNA polymerase, which transcribes the structural genes of the tryptophan operon, leading to the production of enzymes that synthesize tryptophan. When tryptophan becomes available, the enzymes of the tryptophan synthetic pathway are no longer needed, and tryptophan (co­repressor) repressor complex blocks transcription. The regulation of this operon is also a negative control.