The nifH promoter region of Rhizobium leguminosarum: nucleotide sequence and promoter elements controlling activation by NifA protein.

The nucleotide (nt) sequence of the Rhizobium leguminosarum nifH promoter region contains a consensus promoter, a consensus upstream activator sequence (UAS), a pseudo (psi) promoter and a psi UAS. We mapped the transcription start point for the consensus promoter sequence by primer extension. This promoter differs from the consensus in one of the four supposedly invariant nt and can be activated by the Klebsiella pneumoniae nifA product in Escherichia coli. Under these conditions the psi promoter and psi UAS do not function. A low-copy-number plasmid construct containing the psi UAS as well as the consensus UAS delayed the onset of symbiotic nitrogen fixation in nodules induced on Pisum sativum. Studies of high-copy-number nifH promoter constructs showed that partial deletion of the consensus UAS does not alter the ability to inhibit nitrogen fixation by titration of NifA suggesting that NifA can also complex with RNA polymerase containing the alternative sigma-factor RpoN.