Diversity and regulation of ATP sulfurylase in photosynthetic organisms

ATP sulfurylase (AIRS) catalyzes the first committed step in the sulfate assimilation pathway, the activation of sulfate prior to its reduction. AIRS has been studied in only a few model organisms and even in these cases to a much smaller extent than the sulfate reduction and cysteine synthesis enzymes. This is possibly because the latter were considered of greater regulatory importance for sulfate assimilation. Recent evidences (reported in this paper) challenge this view and suggest that AIRS may have a crucial regulatory role in sulfate assimilation, at least in algae. In the ensuing text, we summarize the current knowledge on AIRS, with special attention to the processes that control its activity and gene(s) expression in algae. Special attention is given to algae AIRS proteins. The focus on algae is the consequence of the fact that a comprehensive investigation of AIRS revealed that the algal enzymes, especially those that are most likely involved in the pathway of sulfate reduction to cysteine, possess features that are not present in other organisms. Remarkably, algal AIRS proteins show a great diversity of isoforms and a high content of cysteine residues, whose positions are often conserved. According to the occurrence of cysteine residues, the AIRS of eukaryotic algae is closer to that of marine cyanobacteria of the genera Synechococcus and Prochlorococcus and is more distant from that of freshwater cyanobacteria. These characteristics might have evolved in parallel with the radiation of algae in the oceans and the increase of sulfate concentration in seawater.