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J. Biol. Chem., Vol. 282, Issue 31, 22930-22938, August 3, 2007

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The Putative Moss 3'-Phosphoadenosine-5'-phosphosulfate Reductase Is a Novel Form of Adenosine-5'-phosphosulfate Reductase without an Iron-Sulfur Cluster^*

Stanislav Kopriva¹, Kai Fritzemeier, Gertrud Wiedemann^¶, and Ralf Reski^¶

From the Department of Metabolic Biology, John Innes Centre, Norwich NR4 7UH, United Kingdom and the Plant Biochemistry and ^¶Plant Biotechnology Sections, Faculty of Biology, University of Freiburg, Schaenzlestrasse 1, 79104 Freiburg, Germany

Sulfate assimilation provides reduced sulfur for synthesis of the amino acids cysteine and methionine and for a range of other metabolites. Sulfate has to be activated prior to reduction by adenylation to adenosine 5'-phosphosulfate (APS). In plants, algae, and many bacteria, this compound is reduced to sulfite by APS reductase (APR); in fungi and some cyanobacteria and -proteobacteria, a second activation step, phosphorylation to 3'-phosphoadenosine 5'-phosphosulfate (PAPS), is necessary before reduction to sulfite by PAPS reductase (PAPR). We found previously that the moss Physcomitrella patens is unique among these organisms in possessing orthologs of both APR and PAPR genes (Koprivova, A., Meyer, A. J., Schween, G., Herschbach, C., Reski, R., and Kopriva, S. (2002) J. Biol. Chem. 277, 32195-32201). To assess the function of the two enzymes, we compared their biochemical properties by analysis of purified recombinant proteins. APR from Physcomitrella is very similar to the well characterized APRs from seed plants. On the other hand, we found that the putative PAPR preferentially reduces APS. Sequence analysis, analysis of UV-visible spectra, and determination of iron revealed that this new APR, named PpAPR-B, does not contain the FeS cluster, which was previously believed to determine the substrate specificity of the otherwise relatively similar enzymes. The lack of the FeS cluster in PpAPR-B catalysis is connected with a lower turnover rate but higher stability of the protein. These findings show that APS reduction without the FeS cluster is possible and that plant sulfate assimilation is predominantly dependent on reduction of APS.

Received for publication, March 23, 2007 , and in revised form, May 18, 2007.

^* This work was supported by Deutsche Forschungsgemeinschaft Grant KO2065/3 (Research Group FOR 383 "Sulfur Metabolism in Plants: Junction of Basic Metabolic Pathways and Molecular Mechanisms of Stress Resistance"). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 44-1603-450-276; Fax: 44-1603-450-014; E-mail: stanislav.kopriva{at}bbsrc.ac.uk.

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