Interlink between ExoD (Alr2882), exopolysaccharide synthesis and metal tolerance in Nostoc sp. strain PCC 7120: Insight into its role, paralogs and evolution

Exopolysaccharides (EPS) produced by bacterial species are an important component of bacteria's survival strategy. Synthesis of EPS, principal component of extracellular polymeric substance, occurs through multiple pathways involving multitude of genes. While stress-induced concomitant increase in exoD transcript levels and EPS content have been shown earlier, experimental evidence for direct correlation is lacking. In the present study, role of ExoD in Nostoc sp. strain PCC 7120 was evaluated by generating a recombinant Nostoc strain AnexoD+, wherein the ExoD (Alr2882) protein was constitutively overexpressed. AnexoD+ exhibited higher EPS production, propensity for formation of biofilms and tolerance to Cd stress compared to vector control AnpAM cells. Both Alr2882 and its paralog All1787 exhibited 5 transmembrane domains, with only All1787 predicted to interact with several proteins in polysaccharide synthesis. Phylogenetic analysis of orthologs of these proteins across cyanobacteria indicated that the two paralogs Alr2882 and All1787 and their corresponding orthologs arose divergently during evolution, and could have distinct roles to perform in the biosynthesis of EPS. This study has thrown open the possibility of engineering overproduction of EPS and inducing biofilm formation through genetic manipulation of EPS biosynthesis genes in cyanobacteria, thus building a cost-effective green platform for large scale production of EPS.