Positional requirements for the function of nif-specific upstream activator sequences.

The upstream activator sequence (UAS) found in Klebsiella pneumoniae nif promoters and required for the activation of transcription by nifA, is absent from the nifF-nifL intergenic region, but is present downstream from the nifLA transcription start at +59. To determine whether nif upstream activator sequences can function in a 3' position, the nifH UAS was cloned downstream from the NifH transcription start, but no activation of transcription by nifA dependent upon the UAS in its 3' location could be detected. A mild repressive effect was detectable when the nifH UAS was placed downstream of the nifH promoter, but not when the cat promoter was substituted for the nifLA promoter upstream from the motif at +59 described above. However, deletion analysis showed that the UAS motif located downstream of the nifLA promoter has a role in transcription from the nifF promoter, although it is situated at position -263 with respect to the nifF transcription start, about 100 bp further upstream than previously described occurrences of the activator sequence.