CLONING, SEQUENCING, AND REGULATION OF THE GLOBAL NITROGEN REGULATOR GENE NTCA IN THE UNICELLULAR DIAZOTROPHIC CYANOBACTERIUM CYANOTHECE SPSTRAIN BH68K

In cyanobacteria, ammonium represses expression of proteins involved i n nitrogen fixation and assimilation. The global nitrogen regulator ge ne 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 cy anobacterium, Cyanothece sp. strain BH68K. The gene comprises 678 nucl eotides, and the deduced NtcA protein contains 226 amino acids with a predicted molecular weight of 25,026. In addition, ntcA mRNA levels we re measured in cells grown under different nitrogen regimes. Under nit rogen-fixing conditions, ntcA transcripts were weakly expressed. Furth ermore, ntcA expression was diminished or inversely proportional to ni fHDK expression. Conversely, ntcA expression increased in nitrate-grow n cells, and a concentration-dependent increase was seen in ammonium-g rown cells up to 1 mM NH4Cl. These results indicate that ntcA is invol ved more in nitrogen assimilation than in nitrogen fixation and also i mply that the rhythmic expression of ntcA and nifHDK transcription may be under the control of a circadian clock.