Transcription from Different Promoters Maintains Expression of the Glutamine Synthetase Structural Gene in Anabaena Growing on Ammonia or Fixing Nitrogen

Abstract

We have been interested in the regulatory circuits controlling transcription of the genes for nitrogen fixation and assimilation in cyanobacteria (blue-green algae). Glutamine synthetase (coded by the glnA gene) is a key enzyme in nitrogen assimilation in all organisms. In the heterocyst-forming Anabaena 7120, nitrogen reduced to ammonia by nitrogenase is incorporated into glutamine by glutamine synthetase. The glutamine is then transported into the vegetative cells to serve in the biosynthesis of various metabolites. Glutamine synthetase is needed whether Anabaena cells are growing on ammonia or fixing nitrogen; thus the enzyme is present in the vegetative cells as well as in the heterocysts.