Photosynthetic Electron Transport Controls Nitrogen Assimilation in Cyanobacteria by Means of Posttranslational Modification of the glnB Gene Product

A glnB gene is identified in the cyanobacterium Synechococcus sp. PCC 7942, and its gene product is found to be covalently modified as a result of imbalance in electron transfer in photosynthesis, where photosystem II is favored over photosystem I. The gene was cloned and sequenced and found to encode a polypeptide of 112 amino acid residues, whose sequence shows a high degree of similarity to the Escherichia coli regulatory protein, PII. In E. coli, PIIis involved in signal transduction in transcriptional and post-translational regulation of nitrogen assimilation. Increase in ammonium ion concentration is shown to decrease covalent modification of the Synechococcus PIIprotein, as in enteric bacteria. We therefore propose that the photosynthetic electron transport chain may regulate the pathway of nitrogen assimilation in cyanobacteria by means of posttranslational, covalent modification of the glnB gene product. The existence of the glnB gene in different strains of cyanobacteria is demonstrated and its implications are discussed.