Marine picocyanobacteria are highly abundant primary producers, underpinning the entire marine food web. They are responsible for carbon fixation and produce oxygen through photosynthesis. Genomic and proteomic studies suggest potential mixotrophic capabilities, allowing them to utilise both inorganic and organic compounds. This challenges the traditional paradigm of picocyanobacteria as obligate photoautotrophs. Their streamlined genomes encode diverse predicted nutrient uptake systems for inorganic as well as organic nutrients. Most nutrient uptake in picocyanobacteria is performed by ATP-binding cassette (ABC) transporters associated with substrate-binding proteins (SBP), determining their substrate specificities. Therefore, the SBPs can be used as proxies to understand the functional role of the ABC uptake family in picocyanobacterial nutrient acquisition.
This research aims to characterise a predicted amino acid binding protein encoded in Synechococcus CC9311 to investigate its role in picocyanobacterial niche adaptation. Contrary to predictions, this protein shows no detectable affinity for amino acids but exhibits strong binding to sodium formate and ammonium formate, and accordingly hereby referred to as formate binding protein (CC9311_FBP). The in silico predicted structure of CC9311_FBP also identified a formic acid binding protein (PDB: 4KV7) as the closest structural homolog, with a near identical binding cavity. Furthermore, Synechococcus CC9311 cells supplemented with sodium formate show higher photosynthetic efficiency, suggesting an ability to uptake formate by CC9311 cells. Genomic analysis further reveals Synechococcus CC9311 encodes a putative oxidoreductase, which could potentially function as a formate dehydrogenase to oxidise the formate into CO2. Our ongoing transcriptomic and proteomic studies aim to further investigate the metabolic role of CC9311_FBP in Synechococcus CC9311.
This study identifies CC9311_FBP as a novel formate-binding protein, emphasising the necessity of scrutinising gene functions empirically. It sheds light on nutrient acquisition pathways and advocates for a comprehensive exploration of SBPs in picocyanobacteria, crucial for deciphering their intricate roles in niche adaptation.