Genome-Wide Assessment of Putative Superoxide Dismutases in Unicellular and Filamentous Cyanobacteria

ABSTRACT

Abstract Cyanobacteria are photoautotrophic prokaryotes capable to grow in diverse ecological habitats, originated 2.5-3.5 billion years ago and were first to produce oxygen. Since then superoxide dismutases (SOD) acquired great significance due to their ability to catalyze detoxification of byproducts of oxygenic photosynthesis i.e. superoxide radicals. In the present study, we extracted information regarding SODs from species of sequenced cyanobacteria and investigated their diversity, conservation, domain structure, and evolution. 144 putative SOD homologs were identified. Unlike other protein families (ex. serine-threonine kinases) SODs are present in all cyanobacterial species reflecting their significant role in survival. However, their distribution varies fewer (0.01%-0.09%) found in unicellular marine strains whereas abundant (0.02%-0.07%) in filamentous nitrogen-fixing cyanobacteria. They were classified into three major subfamilies according to their domain structures Fe/MnSOD, Cu/ZnSOD and NiSOD. Interestingly, they lack additional domains as found in proteins of other families however motifs and invariant amino acids typical in eukaryotic SODs were conserved well in these proteins indicating similar catalytic mechanism as eukaryotic SODs. Phylogenetic relationships correspond well with phylogenies based on 16S rRNA and clustering occurs on the basis of structural characteristics such as domain organization. Gene gain-and-loss is insignificant during SOD evolution as evidenced by the absence of additional domain. This study has not only examined an overall background of sequence-structure-function interactions for the SOD gene family but also revealed variation among SOD distribution based on ecophysiological and morphological characters.