Purification and properties of guanosine 5',3'-polyphosphate synthetase from Bacillus brevis

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Abstract
A ribosome-independent guanosine 5'',3''-polyphosphate synthetase was highly purified from B. brevis (ATCC 8185). The enzyme has a MW of 55,000, as measured by sucrose density gradient centrifugation. Like the ribosome-connected stringent factor of Escherichia coli, it catalyzes the synthesis of the guanosine 5'', 3''-polyphosphates by a pyrophosphoryl transfer mechanism from ATP to GDP, GTP. It has an apparent Km of 0.14 mM for GDP and 0.77 mM for GTP, and is specific for the guanosine ribonucleotides as pyrophosphoryl acceptors. Several ATP analogs were tested for their ability to donate the pyrophosphoryl group. Mg2+ was required as a counter ion for the nucleotide substrate, but an excess of Mg2+ was inhibitory. The property of the B. brevis enzyme is compared with the ribosome-linked enzyme of E. coli and an extracellular enzyme excreted by several types of Streptomyces reported recently.

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