Assimilatory sulfate reduction in an Escherichia coli mutant lacking thioredoxin activity

An investigation of sulfate reduction in B tsnC*7004, a mutant of Escherichia coli lacking thioredoxin, is reported. Although thioredoxin is indispensable for the adenosine 3'-phosphate 5'-phosphosulfate (PAPS) sulfotransferase reaction under the usual conditions of assay in extracts of wild-type cells, the mutant grew as well as the wild type on sulfate, indicating that sulfate reduction is not rate limiting for growth. Another cofactor for the PAPS sulfotransferase reaction was found in extracts of the mutant that is absent from wild type cells. This cofactor was indistinguishable from thioredoxin in molecular weight but had a slightly different isoelectric point, allowing a separation of the two types of molecules by isoelectric focusing. Whereas electrons from nicotinamide adenine dinucleotide phosphate, reduced form, could be transferred via thioredoxin reductase or via glutathione and glutathione reductase to reduce thioredoxin in extracts of wild-type cells, electrons from nicotinamide adenine dinucleotide, reduced form, could only be transferred to the cofactor of the mutant via glutathione and glutathione reductase. All of the other available mutants blocked in sulfate reduction in E. coli contained normal levels of thioredoxin. The "PAPS reductase" mutant is shown to be blocked in the PAPS sulfotransferase reaction. We conclude that the cofactor found in mutant B tsnC*7004 is probably a mutated thioredoxin with an amino acid substitution that alters the isoelectric point and the reactivity with thioredoxin reductase.