A glutamine synthetase gene from alfalfa (Medicago sativa) has been expressed in Escherichia coli after fusion of bacterial transcription and translation signals to a complete alfalfa glutamine synthetase coding sequence. Synthesis of the alfalfa glutamine synthetase enzyme in Escherichia coli was demonstrated by functional genetic complementation of a glutamine synthetase-deficient mutant and by immunoblotting analysis. These results should facilitate protein engineering and structure-function analysis of the plant enzyme.