Nitrogen control of the nif regulon in Klebsiella pneumoniae: involvement of the ntrA gene and analogies between ntrC and nifA.

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Abstract
The ntrC and nifA gene products of Klebsiella pneumoniae are transcriptional activators involved in general nitrogen control and nif‐specific regulation, respectively. Multicopy plasmids expressing either ntrC or nifA from a foreign promoter were used to study the relationship between these two genes and ntrA. The nifA product substituted for ntrC product in activation of a number of genes including nifLA, hutUH and genes for arginine and proline utilisation. NtrC could not substitute for nifA in transcriptional activation of the nifHDKY operon. In ntrA‐ strains, neither the ntrC nor the nifA product functioned to activate transcription of nif promoters. In vitro transcription/translation studies with plasmid clones demonstrated similar levels of expression of ntrC and nifA in ntr+ and ntrA‐ S‐30 extracts. Hence, lack of activator function in an ntrA mutant indicates that both the ntrC and nifA products require a functional ntrA for activity. When expressed from foreign promoters, both the ntrC and nifA products were active in conditions which would normally repress nif expression. Hence, the ntrA product was apparently not limiting in these conditions.