Investigation of the Roles of Cysteine Desulfurases in the Molybdopterin Synthesis in Escherichia coli

Abstract

Molybdenum cofactors contain molybdenum and sulfur atoms as functionally essential components. These cofactors are required for the activities of the molybdoenzymes. It has been shown that CsdA, one of 3 cysteine desulfurases in Escherichia coli, can efficiently transfer sulfur from l-cysteine to the C-terminal thiocarboxylate of the smaller subunit of molybdopterin synthase in a defined in vitro system to generate the dithiolene group of molybdopterin from precursor Z. Precursor Z is the immediate precursor of molybdopterin in molybdenum cofactor biosynthesis. In this study, we found that a csdA-deletion strain of E. coli does not accumulate compound Z, a direct oxidation product of precursor Z, unlike a ΔmoaD strain. Although a two-dimensional PAGE analysis suggests that csdA is involved in l-cysteine metabolism, an assay of a molybdenum-dependent enzyme, namely, sulfite oxidase, revealed that the enzyme activity was not markedly different among the ΔcsdA, ΔsufS, and wild-type strains. In contrast, the activity of sulfite oxidase was greatly decreased in the ΔiscS strain. These results suggest that CsdA is not essential for the biosynthesis of molybdenum cofactor and IscS is a possible candidate for the physiological sulfur-donating enzyme in molybdopterin biosynthesis.