Plant Adenosine 5-Phosphosulfate Reductase Is a Novel Iron-Sulfur

lyzes the two-electron reduction of adenosine 5-phos-phosulfate to sulfite and AMP, which represents the key step of sulfate assimilation in higher plants. Recombi-nant APRs from both Lemna minor and Arabidopsis thaliana were overexpressed in Escherichia coli and isolated as yellow-brown proteins. UV-visible spectra of these recombinant proteins indicated the presence of iron-sulfur centers, whereas flavin was absent. This re-sult was confirmed by quantitative analysis of iron and acid-labile sulfide, suggesting a [4Fe-4S] cluster as the cofactor. EPR spectroscopy of freshly purified enzyme showed, however, only a minor signal at g 2.01. There-fore, Mössbauer spectra of 57Fe-enriched APR were ob-tained at 4.2 K in magnetic fields of up to 7 tesla, which were assigned to a diamagnetic [4Fe-4S]2 cluster. Thi