The past five years have witnessed great progress in our understanding of the regulation of one of the best studied blocks of anabolic genes in enteric bacteria, the tryptophan operon. It is now clear that the cell uses two separate mechanisms to regulate transcription of the five structural genes of the operon. Control of transcription initiation occurs at a promoterн operator region through the intervention of a negative-acting repressor, as first postulated by Cohen & Jacob (1) in 1959. Much more recently Yaн nofsky and co-workers (2) demonstrated that this regulation is reinforced by an optional transcription tennination site located in the leader region between the promoter-operator and the first structural gene. This second control region has been tenned the attenuator. Attenuators also exist in other amino acid synthetic pathways, so the tryptophan pathway is not unique in this respect.