Characterization of sulfate assimilation in marine algae focusing on the enzyme 5 '-adenylylsulfate reductase

5'-Adenylylsulfate (APS) reductase was characterized in diverse marine alga e. A cDNA encoding APS reductase from Enteromorpha intestinalis (EAPR) was cloned by functional complementation of an Escherichia coli cysH mutant. Th e deduced amino acid sequence shows high homology with APS reductase (APR) from flowering plants. Based on the probable transit peptide cleavage site the mature protein is 45.7 kD. EAPR expressed as a His-tagged recombinant p rotein catalyzes reduced glutathione-dependent reduction of APS to sulfite, exhibiting a specific activity of approximately 40 mu mol min(-1) mg prote in(-1) and Michealis-Menten kinetic constants of approximately 1.4 mM for r educed glutathione and approximately 6.5 mu M for APS. APR activity and exp ression were studied in relation to the production of 3-dimethylsulfoniopro pionate (DMSP), a sulfonium compound produced by many marine algae. A diver se group of DMSP-producing species showed extremely high enzyme activity (u p to 400 times that found in flowering plants). Antibodies raised against a conserved peptide of APR strongly cross-reacted with a protein of 45 kD in several chlorophytes but insignificantly with chromophytes. Ln the chlorop hyte Tetraselmis sp., APR activity varies significantly during the culture cycle and does not follow the changes in cellular DMSP content. However, a positive correlation was found between cell-based APR activity and specific growth rate.