Initiation in vivo at the internal trp p2 promoter of Escherichia coli.

We have identified an RNA transcript initiated in vivo at the internal promoter of the Escherichia coli trp operon. The 5' end of this message overlaps the distal portion of the trpD structural gene, and the startpoint of transcription is the same as that previously determined in vitro. The relative abundance of the primary and secondary promoter transcripts in cells grown under varying conditions confirms previous genetic data suggesting that the p2 promoter is expressed at a low level, but constitutively. Comparison of p2 with a number of other recently identified internal promoters suggests that the primary function of these elements may be to provide a differentially regulated source of transcription for a subset of genes within the operon.