The evolution of glutamate synthase

DNA coding for the ferredoxin-dependent glutamate synthase (EC1.4.7.1) of s pinach chloroplasts has been cloned and sequenced. It consists of 5015 bp a nd starts with the codon for the N-terminal cysteine of the mature protein. Ferredoxin-dependent glutamate synthase is one of the key enzymes in the e arly stages of ammonia assimilation in plants, algae and cyanobacteria. In addition to the ferredoxin-dependent enzyme, there are two other forms of g lutamate synthase, one of which uses NADH as the electron donor and a secon d that uses NADPH. Although all three forms catalyze the reductive transami dation of the amido nitrogen from glutamine to 2-oxoglutarate to form two m olecules of glutamate, ferredoxin-dependent glutamate synthases differ from the NADH and NADPH-dependent forms in subunit composition and amino acid s equence. The recent availability of sequence data for glutamate synthases f rom spinach and from two archael species has produced a clearer and more de tailed picture of the evolution of this key enzyme in nitrogen metabolism a nd the origins of the two subunit/domain structure of the enzyme.