Protein involved in the synthesis of the molybdenum cofactor (Moco), a molybdenum atom coordinated to the sulfur atoms of a pterin derivative (molybdopterin [MPT]). It is highly conserved in eukaryotes, eubacteria, and archaebacteria. In prokaryotes, two operons are directly associated with biosynthesis of the pterin moiety and its side chain while additional loci play a role in the acquisition of molybdenum and/or activation of the cofactor.