Escherichia coli glutamine synthetase, (GS), is a regulatory enzyme with 12 identical subunits (MW 600,000). It requires two divalent metal ions per subunit for optimal activity, and catalyzes the production of L-glutamine from L-glutamate, ATP, and NH(,4)('+). The enzyme is regulated by several end products of glutamine metabolism. Different forms of the enzyme are found in plants, animals, and other bacteria. and GS is isolated from an overproducing strain of E. coli by a method involving Zn-Mg precipitation. The enzyme was found to be homogeneous on polyacrylamide gels by an ultra-sensitive silver staining technique. Solutions of enzyme at 1 mM concentrations in phosphate buffer were found to be relatively stable over a period of several days.