Evidence for an internal promoter in the Escherichia coli threonine operon

We constructed plasmids carrying the two first genes of the threonine operon from which the major promoter was deleted in vitro by digestion with BAL 31 nuclease. These plasmids continued to express the second gene (thrB) of the operon as judged by their ability to complement a threonine auxotroph. These data indicate that, in addition to the major promoter thrP, there was an internal promoter, thrBp, which could be used for the transcription of the thrB, the second gene of the operon. Additional evidence was given by subcloning a 230-base-pair segment of the operon in a plasmid suitable for detection of translation initiation signals and promoters. The thrBp promoter was thus shown to lie within a 61-base-pair fragment at the 3' end of the first gene, thrA, of the threonine operon.