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Bacillus subtilis Glutamine Synthetase

PURIFICATION AND PHYSICAL CHARACTERIZATION

From the ¹ From the Laboratories of Biochemistry, National Heart and Lung Institute and National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20014

A procedure is described for the purification of glutamine synthetase from Bacillus subtilis, grown under conditions of nitrogen limitation, to a nearly homogeneous state. Although the purified enzyme differs from the glutamine synthetase from Escherichia coli in catalytic properties and stability, the physical characteristics of these two enzymes are quite similar. The B. subtilis enzyme has a molecular weight of 600,000, a sedimentation coefficient at infinite dilution of 19.3 S, and is seen in electronmicrographs as a molecule composed of 12 subunits arranged in 2 superimposed hexagonal rings. Studies with the dissociated enzyme suggest that the subunit molecular weight is 50,000, in agreement with a dodecameric aggregate structure of the native enzyme which contains 12 subunits of similar size. Both Mn²⁺ and Mg²⁺ activate the enzyme in the biosynthesis of l-glutamine, but, unlike the E. coli system, the Mg²⁺-dependent activity is intrinsically less stable than the Mn²⁺-dependent activity. Differences between the B. subtilis and E. coli enzymes are apparent also in the amino acid compositions, in the susceptibility to digestion by carboxypeptidase A, in the C-terminal amino acid of the subunit polypeptide chains, in immunochemical properties, and in that the E. coli adenylylating enzyme system does not incorporate 5'-¹⁴C-adenylyl groups into the B. subtilis glutamine synthetase.

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