The diversity and complexity of the cyanobacterial thioredoxin systems

Cyanobacteria perform oxygenic photosynthesis, which makes them unique among the prokaryotes, and this feature together with their abundance and worldwide distribution renders them a central ecological role. Cyanobacteria and chloroplasts of plants and algae are believed to share a common ancestor and the modern chloroplast would thus be the remnant of an endosymbiosis between a eukaryotic cell and an ancestral oxygenic photosynthetic prokaryote. Chloroplast metabolic processes are coordinated with those of the other cellular compartments and are strictly controlled by means of regulatory systems that commonly involve redox reactions. Disulphide/dithiol exchange catalysed by thioredoxin is a fundamental example of such regulation and represents the molecular mechanism for light-dependent redox control of an ever-increasing number of chloroplast enzymatic activities. In contrast to chloroplast thioredoxins, the functions of the cyanobacterial thioredoxins have long remained elusive, despite their common origin. The sequenced genomes of several cyanobacterial species together with novel experimental approaches involving proteomics have provided new tools for re-examining the roles of the thioredoxin systems in these organisms. Thus, each cyanobacterial genome encodes between one and eight thioredoxins and all components necessary for the reduction of thioredoxins. Screening for thioredoxin target proteins in cyanobacteria indicates that assimilation and storage of nutrients, as well as some central metabolic pathways, are regulated by mechanisms involving disulphide/dithiol exchange, which could be catalysed by thioredoxins or related thiol-containing proteins.