Transfers a GMP moiety from GTP to Mo-molybdopterin (Mo-MPT) cofactor (Moco or molybdenum cofactor) to form Mo-molybdopterin guanine dinucleotide (Mo-MGD) cofactor. Is also involved in the biosynthesis of the bis-MGD form of the Moco cofactor (Mo-bisMGD) in which the metal is symmetrically ligated by the dithiolene groups of two MGD molecules. Is necessary and sufficient for the in vitro activation of the DMSOR molybdoenzyme that uses the Mo-bisMGD form of molybdenum cofactor, which implies formation and efficient insertion of the cofactor into the enzyme without the need of a chaperone. Is specific for GTP since other nucleotides such as ATP and GMP cannot be utilized. GTP + H(+) + Mo-molybdopterin = diphosphate + Mo-molybdopterin guanine dinucleotide Both divalent cations appear to be equally efficient in an vitro reconstitution assay. Monomer. An equilibrium exists between a monomeric and oligomeric form of the enzyme, which could be an octamer; whether this oligomeric arrangement is of functional relevance is unclear. Interacts with MoeA and MobB in vivo. Is expressed at very low levels under both aerobic and anaerobic growth conditions. The N-terminal domain determines nucleotide recognition and specific binding, while the C-terminal domain determines the specific binding to the target protein. When the N-terminal domain of MobA is fused to the C-terminal domain of MocA, comparable kinetic constants as wild-type MobA are obtained with GTP, and the activity with CTP is completely lost. Consistent results are obtained when the N-terminal domain of MocA is fused to the C-terminal domain of MobA: the kinetic constants with CTP are comparable with the ones found for wild-type MocA, although no activity with GTP is detected. Cells lacking this gene are chlorate-resistant, fail to synthesize MGD and accumulate elevated quantities of MPT. Belongs to the MobA family.