ATP sulfurylase (ATPS) catalyzes the first step of sulfur assimilation in photosynthetic organisms. An ATPS type A is mostly present in freshwater cyanobacteria, with four conserved cysteine residues. Oceanic cyanobacteria and most eukaryotic algae instead, possess an ATPS-B containing seven to ten cysteines; five of them are conserved, but only one in the same position as ATPS-A. We investigated the role of cysteines on the regulation of the different algal enzymes. We found that the activity of ATPS-B from four different microorganisms was enhanced when reduced and decreased when oxidized. The LC-MS/MS analysis of the ATPS-B from the marine diatom Thalassiosira pseudonana showed that the residue Cys-247 was presumably involved in the redox regulation. The absence of this residue in the ATPS-A of the freshwater cyanobacterium Synechocystis sp. instead, was consistent with its lack of regulation. Some other conserved cysteine residues in the ATPS from T. pseduonana and not in Synechocystis sp.were accessible to redox agents and possibly play a role in the enzyme regulation. Furthermore, the fact that oceanic cyanobacteria have ATPS-B structurally and functionally closer to that from most of eukaryotic algae than to the ATPS-A from other cyanobacteria suggests that life in the sea or freshwater may have driven the evolution of ATPS.

Aix Marseille Univ CNRS BIP UMR 7281 IMM 31 Chemin J Aiguier 13402 Marseille Cedex 20 France

Dipartimento di Scienze della Vita e dell'Ambiente Università Politecnica delle Marche via Brecce Bianche 60131 Ancona Italy

Institute of Microbiology ASCR Algatech Trebon Czech Republic

National Research Council Institute of Marine Science Venezia Italy

Plate forme Protéomique MaP IMM FR 3479 CNRS 31 Chemin J Aiguier 13402 Marseille Cedex 20 France

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