The nif promoters of Klebsiella pneumoniae have a characteristic primary structure.

We have determined the precise point of transcription initiation for five nif (nitrogen fixation) operons of Klebsiella pneumoniae and sequenced the promoters. Our results show that nifF is transcribed in the opposite direction to that previously proposed for nif genes, that nifM is transcribed from two promoters, and that there is a promoter preceding nifU but not nifX, and we present a revised map of nif transcripts. The nif promoters have a characteristic structure of 26 bp located between positions--1 and --26 upstream of the site of transcription initiation: CTGG..8bp...TTGCA...9bp...Py(+1). This structure, which has two regions of conserved sequence, shows no homology to the "consensus promoter" of enteric bacteria. Our results suggest that the--10 homology is equivalent to a "Pribnow box" for promoters expressed under nitrogen-starved conditions and that the--23 homology confers activator specificity on the nif promoters.