The use of cloned nif (nitrogen fixation) DNA to investigate transcriptional regulation of nif expression in Klebsiella pneumoniae.

Some restriction endonuclease fragments of nif DNA, when carried on small multicopy plasmids, inhibited nif expression in Klebsiella pneumoniae. A study of this inhibitory effect revealed, (1) that overproduction of the nifL gene product inhibited transcription of two nif operons examined, nifJ and nifHDKY and, (2) that when transcription was initiated from the promoter of the nifHDKY operon on multicopy plasmids there was a corresponding decrease in the transcription rates of the chromosomally located nifJ and nifHDKY but not the nifLA operon. Studies of transcription in vivo also showed that the nifA gene product was essential for transcription initiation from the nifHDKY and nifBQ promoters. These results, taken with earlier observations (see Discussion) provide evidence that the nifL and nifA gene products are respectively a repressor and activator of nif transcription initiation from all nif promoters except that of the nifLA operon.