In the context of the tryptophan operon, what signifies that gene expression is 'ON'?

In the context of the tryptophan operon, gene expression is signified as 'ON' when RNA polymerase (RNAP) is bound to the promoter region of the operon. This binding allows the transcription of the genes involved in the biosynthesis of tryptophan to occur. When RNAP is attached to the promoter, it initiates the process of transcription, which leads to the production of mRNA and subsequently the synthesis of the enzymes needed for tryptophan synthesis.

The tryptophan operon is regulated through a mechanism involving the trp repressor, which binds to the operator region when tryptophan levels are high, effectively blocking RNAP from transcribing the genes. Therefore, when RNAP is bound, it indicates that tryptophan levels are low, allowing for gene expression to proceed, which aligns with the operon's role in responding to nutrient availability.

Thus, the correct answer reflects the state in which the genes of the operon are actively being transcribed, marking a crucial aspect of post-translational regulation in response to tryptophan levels.