Regulation and Properties of the Glutamine Synthetase Purified from Photobacterium phosphoreum

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Abstract
Glutamine synthetase from a marine enterobacterium, Photobacterium phosphoreum, was purified to homogeneity from cells grown in glycerol-yeast extract medium. The purified enzyme had a molecular weight of approximately 670,000 and a sub-unit size of 56,000, i.e. larger than that of the enzyme from E. coli. Regulation of the glutamine synthetase activity by adenylylation/deadenylylation was demonstrated on snake venom phosphodiesterase treatment. The state of adenylylation appeared to influence both the biosynthetic and γ-glutamyltransferase activities of P. phosphoreum glutarnine synthetase similar to in the case of the E. coil enzyme. The enzyme activity was controlled by adenylylation and possibly in combination with feedback inhibition by alanine, serine, and glycine, metabolites which are especially effective in inhibiting P. phosphoreum glutamine synthetase. When either Mn2+ or Mg2+ was added to the relaxed (divalent cation-free) enzyme, similar UV-difference spectra were obtained for the enzyme, indicating that the conformational states induced by these cations were also similar. The proffle of these spectra varied from those published for E. coil, and three peaks were found at 282.5, 288.5, and 298 nm.