Determining the real function of a putative cyanobacterial amino acid binding protein

<p dir="ltr">Marine picocyanobacteria underpin the entire marine food web, recycling nutrients like carbon, nitrogen and phosphorus. Their streamlined genomes encode predicted nutrient acquisition systems, including ATP Binding Cassette (ABC) transporters for inorganic and organic nutrients. Substratebinding proteins (SBPs) primarily function with ABC transporters, determining their substrate specificities. They can, therefore, serve as proxies to understand picocyanobacterial nutrient acquisition systems.</p><p dir="ltr">This study investigated the functional characteristics of a predicted amino acid binding protein (CC9311_SC634) in <i>Synechococcus </i>CC9311 to understand its role in picocyanobacterial niche adaptation. Ligand screening and binding affinity assays revealed that CC9311_SC634 has a strong affinity for formate over amino acids. Predicted structure comparison with a known formic acidbinding protein (PDB: 4KV7) supported this unexpected finding. Cell physiology experiments showed increased photosynthetic efficiency in CC9311 cells in the presence of formate.</p><p dir="ltr">This study reveals CC9311_SC634 as a formate-binding protein in contrast to its predicted function from bioinformatic analyses. The experimental work in this thesis unravelled a previously unknown function, emphasising the necessity of scrutinising gene functions empirically. This study not only sheds light on a specific protein but advocates for a comprehensive exploration of SBPs in picocyanobacteria, crucial for deciphering their intricate roles in nutrient acquisition.</p>