Positive and negative control of the glnA ntrBC regulon in Klebsiella pneumoniae.

The nitrogen regulation system of Klebsiella pneumoniae comprises three genes ntrA, ntrB and ntrC. We have found that the glnA ntrBC regulon in K. pneumoniae has a similar structure, P1 glnA P2 ntrBC, to that in other enterobacteria. We have constructed plasmids with glnA and ntrB translational lacZ fusions and measured expression from P1 and/or P2 in a K. pneumoniae delta (glnA ntrBC) background with different plasmids which provided the ntrB, ntrC or nifA products in trans. These studies demonstrate that, as in other enterobacteria, transcription of ntrBC is from P1 under nitrogen deficiency and from P2 under nitrogen excess. The P1 promoter can be regulated both positively and negatively; activation requires both ntrB and ntrC products but the ntrC product is sufficient to repress. The P2 promoter is negatively controlled by the ntrC product. Comparison of the modes of regulation of P1 and P2 with regulation of the promoter of the nifLA operon leads us to suggest that these may represent three different classes of ntr-regulated promoters. Although previous studies have shown that the nifA product can substitute for the ntrC product as a positive activator of transcription for a number of promoters, we find that nifA product cannot substitute for ntrC product as a negative regulator at P1 or P2.