A MUTANT LACKING THE GLUTAMINE-SYNTHETASE GENE (GLNA) IS IMPAIRED IN THE REGULATION OF THE NITRATE ASSIMILATION SYSTEM IN THE CYANOBACTERIUM SYNECHOCYSTIS SP STRAIN PCC-6803

The existence in the unicellular cyanobacterium Synechocystis sp. stra in PCC 6803 of two genes (glnA and glnN) coding for glutamine syntheta se (GS) has been recently reported (J. C. Reyes and F. J. Florencio, J . Bacteriol. 176:1260-1267, 1994). In the current work, the regulation of the nitrate assimilation system was studied with a glnA-disrupted Synechocystis mutant (strain SJCR3) in which the only GS activity is t hat corresponding to the glnN product. This mutant was unable to grow in ammonium containing medium because of its very low levels of GS act ivity. In the SJCR3 strain, nitrate and nitrite reductases were not re pressed by ammonium, and short-term ammonium-promoted inhibition of ni trate uptake was impaired. In Synechocystis sp. strain PCC 6803, nitra te seems to act as a true inducer of its assimilation system, in a way similar to that proposed for the dinitrogen-fixing cyanobacteria. A s pontaneous derivative strain from SJCR3 (SJCR3.1), was able to grow in ammonium-containing medium and exhibited a fourfold-higher level of G S activity than but the same amount of glnN transcript as its parental strain (SJCR3). Taken together, these finding suggest that SJCR3.1 is a mutant affected in the posttranscriptional regulation of the GS enc oded by glnN. This strain recovered regulation by ammonium of nitrate assimilation. SJCR3 cells were completely depleted of intracellular gl utamine shortly after addition of ammonium to cells growing with nitra te, while SJCR3.1 cells maintained glutamine levels similar to that re ached in the wild-type Synechocystis sp. strain PCC 6803. Our results indicate that metabolic signals that control the nitrate assimilation system in Synechocystis sp. strain PCC 6803 require ammonium metabolis m through GS.