Cyanobacteria are photosynthetic microorganisms ubiquitous across aquatic environments, significantly impacting the marine food web as primary producers. The picocyanobacterial lineages of Prochlorococcus and Synechococcus each divide into clades with distinct spatio-temporal distributions around the globe. Their relatively streamlined genomes harbour nutrient uptake systems suggestive of mixotrophy, i.e. both the ability to harvest light for direct biosynthesis from inorganic components, as well as capacity to assimilate organic nutrients from a surrounding marine environment. We are engaged in a molecules-to-ecosystems investigation of this little understood nutrient acquisition process.
A surprising prevalence of solute/substrate binding proteins (SBPs) is evident in our sequence analysis of model strains of Synechococcus. SBP proteins are known as high-affinity binding components for the uptake of diverse nutrients (e.g. inorganics, carbohydrates, amino acids), generally through partnership with specific ABC membrane transporters. We outline progress with our functional and structural characterization of over 300 predicted SBPs within cyanobacteria from varied ecological niches. Recombinant forms produced in fast-throughput formats are being subjected to crystallization screening and thermal shift assays, for structure determination and substrate identification, respectively. We report on SBP crystal structures solved to date, and associated binding studies. Our findings are refining the existing crude classifications of the SBP superfamily, as well as postulating new nutrient uptake models for marine cyanobacteria.