ID 2.7.7.45 DE guanosine-triphosphate guanylyltransferase. CA 2 GTP + H(+) = diphosphate + P(1),P(4)-bis(5'-guanosyl) tetraphosphate. CC -!- Also acts, more slowly, on GDP to form P(1),P(3)-bis(5'-guanosyl) CC triphosphate. //