Ecological aspects of ntcA gene expression and its use as an indicator of the nitrogen status of marine Synechococcus spp.

Nitrogen nutrition in cyanobacteria is regulated by NtcA, a transcriptional activator that is subject to negative control by ammonium. Using Synechoco ccus sp. strain WH7803 as a model organism, we show that ntcA expression wa s induced when cells were exposed to nitrogen stress but not when they were subjected to phosphorus or iron deprivation. Transcript levels accumulated in cells grown on a variety of inorganic and organic nitrogen sources, wit h the sole exception of ammonium. ntcA transcription was induced when ammon ium levels dropped below 1 muM and reached maximal levels within 2 h. Furth ermore, the addition of more than I muM ammonium led to a rapid decline in ntcA mRNA. The negative effect of ammonium was prevented by the addition of L-methionine-D,L-Sulfoximine (MSX) and azaserine, inhibitors of ammonium a ssimilation. Thus, basal ntcA transcript levels are indicative of ammonium utilization. Conversely, the highest ntcA transcript levels were found in c ells lacking a nitrogen source capable of supporting growth. Therefore, max imal ntcA expression would indicate nitrogen deprivation. This state of nit rogen deprivation was induced by a 1-h incubation with MSX. The rapid respo nse of ntcA gene expression to the addition of ammonium and MSX was used to design a protocol for assessing relative ntcA transcript levels in field p opulations of cyanobacteria, from which their nitrogen status can be inferr ed. ntcA was basally expressed in Synechococcus at a nutrient-enriched site at the northern tip of the Gulf of Aqaba, Red Sea. Therefore, these cyanob acteria were not nitrogen stressed, and their nitrogen requirements were me t by regenerated nitrogen in the form of ammonium.