Phycobilisomes (PBS) are the intricate light-harvesting antennas found in cyanobacteria. To balance the harvesting of light energy against the risks of photodamage, many cyanobacteria have evolved a photoprotective mechanism that relies on the interaction between a photoreceptor, the Orange Carotenoid Protein (OCP), and the PBS. Recently, the PBS and the complex OCP-PBS structure have been elucidated from the model organism Synechocystis PCC 6803 at overall resolutions 1.6- 3.5 Å. The structures revealed the existence of three different conformational states of the antenna, including two previously unknown for the unquenched PBS. The PBS-OCP complex showed four OCPs organized as two dimers that quench the PBS. In our current work, we are biochemically characterizing OCP from three different marine Synechococcus strains from various ecological environments in the ocean. Additionally, we are analyzing gene expression under different conditions, such as high light or darkness, and identifying and quantifying pigment content, with a special focus on carotenoids, under these conditions. To date, OCP proteins have been well studied in freshwater cyanobacterial models; however, little is known about marine OCPs. Our bioinformatics analysis revealed that OCP from marine Synechococcus is relatively poorly conserved, with only 65% sequence identity to the OCP1 of freshwater strains (typically 85-90% identical), making them the most divergent OCP. We hypothesized that differences in primary structure would be reflected in photoconversion kinetics.
Recently, new homologous families of the constituent domains of OCP have been identified (Melnicki et al. 2016). Nine different clades of N-terminal domain homologs have been described across diverse cyanobacteria species and are named Helical Carotenoid Proteins (HCPs). Homologs to the C-terminal domain (CCPs) have also been found in nearly every genome encoding an HCP. Most likely, OCP was derived from a combination of HCP with CCP forming a single polypeptide. We are characterizing the OCP-related system of another marine cyanobacteria, Cyanothece ATCC 51142.
Altogether, our on-going research will shed light on the regulation of photosynthesis and photoprotection in these ecologically relevant species.