Cloning, sequencing, and regulation of the global nitrogen regulator gene ntcA in the unicellular diazotrophic cyanobacterium Cyanothece sp. strain BH68K.

In cyanobacteria, ammonium represses expression of proteins involved in nitrogen fixation and assimilation. The global nitrogen regulator gene ntcA encodes a DNA-binding protein, NtcA, that is a transcriptional activator of genes subject to nitrogen control. We report the cloning and sequencing of the ntcA gene from a nitrogen-fixing unicellular cyanobacterium, Cyanothece sp. strain BH68K. The gene comprises 678 nucleotides, and the deduced NtcA protein contains 226 amino acids with a predicted molecular weight of 25,026. In addition, ntcA mRNA levels were measured in cells grown under different nitrogen regimes. Under nitrogen-fixing conditions, ntcA transcripts were weakly expressed. Furthermore, ntcA expression was diminished or inversely proportional to nifHDK expression. Conversely, ntcA expression increased in nitrate-grown cells, and a concentration-dependent increase was seen in ammonium-grown cells up to 1 mM NH4Cl. These results indicate that ntcA is involved more in nitrogen assimilation than in nitrogen fixation and also imply that the rhythmic expression of ntcA and nifHDK transcription may be under the control of a circadian clock.