POSITIVE REGULATION BY NITRITE OF THE NITRATE ASSIMILATION OPERON IN THE CYANOBACTERIA SYNECHOCOCCUS SP STRAIN PCC-7942 AND PLECTONEMA-BORYANUM

In the absence of fixation of ammonium to glutamine, nitrate and nitri te activated transcription of the nitrate assimilation (nirA-nrtABCD-n arB) operon of Synechococcus sp. strain PCC 7942. In a nitrate reducta se-deficient mutant, only nitrite activated transcription, indicating that nitrite is the actual activator of the operon. Nitrate and nitrit e were also found to activate the transcription of a nitrate assimilat ion operon in the filamentous nonheterocystous nitrogen-fixing cyanoba cterium Plectonema boryanum.