Nitrogen control of the glnN gene that codes for GS type III, the only glutamine synthetase in the cyanobacterium Pseudanabaena sp. PCC 6903

Pseudanabaena sp. strain PCC 6903 is the first cyanobacteria lacking the ty pical prokaryotic glutamine synthetase type I encoded by the glnA gene. The glnN gene product, glutamine synthetase type ill, is the only glutamine sy nthetase activity present in this cyanobacterium. Analysis of glnN expressi on clearly indicated a nitrogen-dependent regulation. Pseudanabaena glnN ge ne expression and GSIII activity were upregulated under nitrogen starvation or using nitrate as a nitrogen source, while low levels of transcript and activity were found in ammonium-containing medium. Primer extension analysi s showed that the glnN gene promoter structure resembled that of the NtcA-r elated promoters. Mobility shift assays demonstrated that Synechocystis sp. PCC 6803 NtcA protein, expressed and purified from Escherichia coli, bound to the promoter of the Pseudanabaena 6903 glnN gene. The NtcA control of t he glnN gene in this cyanobacterium suggested that, in the absence of a gln A gene, NtcA took control of the only glutamine synthetase gene in a fashio n similar to the way the glnA gene is governed in those cyanobacteria harbo uring a glnA gene.