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Journal of Bacteriology, December 2006, p. 8551-8559, Vol. 188, No. 24
0021-9193/06/$08.00+0     doi:10.1128/JB.00987-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Malonyl-Coenzyme A Reductase in the Modified 3-Hydroxypropionate Cycle for Autotrophic Carbon Fixation in Archaeal Metallosphaera and Sulfolobus spp.

Mikrobiologie, Institut für Biologie II, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany

Received 6 July 2006/ Accepted 3 October 2006

Autotrophic members of the Sulfolobales (Crenarchaeota) contain acetyl-coenzyme A (CoA)/propionyl-CoA carboxylase as the CO₂ fixation enzyme and use a modified 3-hydroxypropionate cycle to assimilate CO₂ into cell material. In this central metabolic pathway malonyl-CoA, the product of acetyl-CoA carboxylation, is further reduced to 3-hydroxypropionate. Extracts of Metallosphaera sedula contained NADPH-specific malonyl-CoA reductase activity that was 10-fold up-regulated under autotrophic growth conditions. Malonyl-CoA reductase was partially purified and studied. Based on N-terminal amino acid sequencing the corresponding gene was identified in the genome of the closely related crenarchaeum Sulfolobus tokodaii. The Sulfolobus gene was cloned and heterologously expressed in Escherichia coli, and the recombinant protein was purified and studied. The enzyme catalyzes the following reaction: malonyl-CoA + NADPH + H⁺ malonate-semialdehyde + CoA + NADP⁺. In its native state it is associated with small RNA. Its activity was stimulated by Mg²⁺ and thiols and inactivated by thiol-blocking agents, suggesting the existence of a cysteine adduct in the course of the catalytic cycle. The enzyme was specific for NADPH (K_m = 25 µM) and malonyl-CoA (K_m = 40 µM). Malonyl-CoA reductase has 38% amino acid sequence identity to aspartate-semialdehyde dehydrogenase, suggesting a common ancestor for both proteins. It does not exhibit any significant similarity with malonyl-CoA reductase from Chloroflexus aurantiacus. This shows that the autotrophic pathway in Chloroflexus and Sulfolobaceae has evolved convergently and that these taxonomic groups have recruited different genes to bring about similar metabolic processes.

Published ahead of print on 13 October 2006.

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