MUTATIONS IN A POLYCISTRONIC NUCLEAR GENE ASSOCIATED WITH MOLYBDENUM COFACTOR DEFICIENCY

All molybdoenzymes other than nitrogenase require molyb-dopterin as a metal-binding cofactor(1). Several genes necessary for the synthesis o f the molybdenum cofactor (MoCo) have been characterized in bacteria(2 ,3) and plants(4). The proteins encoded by the Escherichia coli genes moaA and moaC catalyse the first steps in MoCo synthesis. The human ho mologues of these genes are therefore candidate genes for molybdenum c ofactor deficiency, a rare and fatal disease(5). Using oligonucleotide s complementary to a conserved region in the moaA gene, we have isolat ed a human cDNA derived from liver mRNA. This transcript contains an o pen reading frame (ORF) encoding the human moaA homologue and a second ORF encoding a human moaC homologue. Mutations can be found in the ma jority of MoCo-deficient patients that confirm the functional role of both ORFs in the corresponding gene MOCS1 (for 'molybdenum cofactor sy nthesis-step 1'). Northern-blot analysis detected only full-length tra nscripts containing both consecutive ORFs in various human tissues. Th e mRNA structure suggests a translation reinitiation mechanism for the second ORF, These data indicate the existence of a eukaryotic mRNA, w hich as a single and uniform transcript guides the synthesis of two di fferent enzymatic polypeptides with disease-causing potential.