Transfers a CMP moiety from CTP to Mo-molybdopterin (Mo-MPT) cofactor (Moco or molybdenum cofactor) to form Mo-molybdopterin cytosine dinucleotide (Mo-MCD) cofactor. Is specific for CTP; other nucleotides such as ATP and GTP cannot be utilized. Is also able to convert MPT to MCD in the absence of molybdate, however, with only one catalytic turnover. CTP + H(+) + Mo-molybdopterin = diphosphate + Mo-molybdopterin cytosine dinucleotide Mg(2+) is essential for activity. However, Mn(2+) is able to functionally replace Mg(2+) with a 50% reduced efficiency, whereas no MCD is produced with other divalent cations like Co(2+) or Ni(2+). Monomer. Interacts with the Moco-binding chaperone PaoD. 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. A disruption in the mocA gene impairs MCD biosynthesis in E.coli, resulting in an inactive PaoABC aldehyde oxidoreductase devoid of MCD cofactor.