Promoter for the unc operon of Escherichia coli.

Fragments of DNA carrying possible promoters for the unc operon of Escherichia coli were cloned into a promoter detection plasmid (pRZ5255). Similar fragments were transcribed in vitro to produce transcripts whose sizes were used to determine the approximate start site for transcription. One strong promoter and at least two very much weaker ones were detected by these methods. The exact position of the strongest promoter, presumed to be the true unc promoter, was determined by S1 nuclease mapping and shown to lie 73 base pairs upstream from the open reading frame that precedes uncB. It therefore appears that this reading frame (uncI) is part of the unc operon. S1 mapping also revealed the presence of a third weak promoter 25 base pairs upstream of uncI. All of the weak promoters occur between the proposed unc promoter and uncB, but their role in vivo, if any, is unclear.