Here we report the first discovery of adenosine 5'-phosphosulfate reductase (APR), a key enzyme of the sulfate reduction pathway, in the methanarchaeon Methanocaldococcus jannaschii. While the sulfate reduction pathway is present in other organisms, it is not expected to exist in methanarchaea because their habitats often already possess an abundance of reduced sulfur, particularly H2S. However, the gene product of open reading frame (ORF) Mj0973 in M. jannaschii possesses sequence similarities with known APRs and 3'-phosphoadenosine-5'-phosphosulfate reductases (PAPRs) from various organisms. In order to further investigate this ORF, we expressed the gene Mj0973 from M. jannaschii and purified the resulting protein. Kinetic studies revealed that the purified protein is able to reduce APS with Escherichia coli thioredoxin (Trx) supplied as the electron donor, but is unable to reduce PAPS. The apparent K-m, V-max, and k(cat)/K-m values at pH 8.0 and 30 degrees C were 0.29 mu M, 0.079 mu M mg(-1) min(-1), and 299,655 M-1 s(-1), respectively. This observation of APR activity strongly indicates the presence of an APS-utilizing sulfate reduction pathway in the methanarchaeon M. jannaschii. (C) 2010 Elsevier Ltd. All rights reserved.