Characterisation of the mob locus of Rhodobacter sphaeroides WS8: mobA is the only gene required for molybdopterin guanine dinucleotide synthesis

The mob genes of several bacteria have been implicated in the conversion of molybdopterin to molybdopterin guanine dinucleotide. The mob locus of Rhod obacter sphaeroides WS8 comprises three genes, mobABC. Chromosomal in-frame deletions in each of the mob genes have been constructed. The mobA mutant strain has inactive DMSO reductase and periplasmic nitrate reductase activi ties (both molybdopterin guanine dinucleotide-requiring enzymes), but the a ctivity of xanthine dehydrogenase, a molybdopterin enzyme, is unaffected. T he inability of a mobA mutant to synthesise molybdopterin guanine dinucleot ide is confirmed by analysis of cell extracts of the mobA strain for molybd enum cofactor forms following, iodine oxidation. Mutations in mobB and mobC are not impaired for molybdoenzyme activities and accumulate wild-type lev els of molybdopterin and molybdopterin guanine dinucleotide, indicating the y are not compromised in molybdenum cofactor synthesis. In the mobA mutant strain, the inactive DMSO reductase is found in the periplasm, suggesting t hat molybdenum cofactor insertion is not necessarily a pre-requisite for ex port.