The coding sequence for Anabaena 7120 glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.1] are shown to be contained within a 7.5-kilobase-pair (kbp) HindIII fragment that has been cloned by plaque hybridization. The hybridization probe for the cyanobacterial gene was a recombinant plasmid containing the glnA gene from Escherichia coli K-12. Evidence that the cloned Anabaena fragment contains the glnA gene includes complementation of a glnA deletion mutant of E. coli and immunological identity of the enzyme produced by the cloned Anabaena fragment in E. coli with glutamine synthetase purified from Anabaena 7120. Heteroduplex analysis reveals 0.65 kbp of homology between the 7.5-kbp Anabaena 7120 fragment and an 11-kbp E. coli fragment that codes for E. coli glutamine synthetase. Studies of Anabaena glnA gene activity in E. coli suggest that the cyanobacterial gene is not repressible and that the Anabaena 7120 glutamine synthetase is not adenylylated in E. coli.