Abstract
As a component of the photosynthetic apparatus in cyanobacteria, the phycobilisome (PBS) plays an important role in harvesting and transferring light energy to the core photosynthetic reaction centers. The size, composition (phycobiliprotein and chromophore), and assembly of PBSs can be dynamic to cope with tuning photosynthesis and associated cellular fitness in variable light environments. Here, we explore the role of PBS-related stress responses by analyzing deletion mutants of cpcF or cpcG1 genes in Synechocystis sp. PCC 6803. The cpcF gene encodes a lyase that links the phycocyanobilin (PCB) chromophore to the alpha subunit of phycocyanin (PC), a central phycobiliprotein (PBP) in PBSs. Deletion of cpcF (i.e., ΔcpcF strain) resulted in slow growth, reduced greening, elevated reactive oxygen species (ROS) levels, together with an elevated accumulation of a stress-related Peroxiredoxin protein (Sll1621). Additionally, ΔcpcF exhibited reduced sensitivity to a photosynthesis-related stress inducer, methyl viologen (MV), which disrupts electron transfer. The cpcG1 gene encodes a linker protein that serves to connect PC to the core PBP allophycocyanin. A deletion mutant of cpcG1 (i.e.,ΔcpcG1) exhibited delayed growth, a defect in pigmentation, reduced accumulation of ROS, and insensitivity to MV treatment. By comparison, ΔcpcF and ΔcpcG1 exhibited similarity in growth, pigmentation, and stress responses; yet, these strains showed distinct phenotypes for ROS accumulation, sensitivity to MV and Sll1621 accumulation. Our data emphasize an importance of the regulation of PBS structure in ROS-mediated stress responses that impact successful growth and development in cyanobacteria.
Highlights
Phycobilisomes (PBSs) are abundant light-harvesting protein complexes, which comprise up to 60% of the total protein content of cyanobacteria (Singh et al, 2015)
We demonstrated the effect of deletion of cpcF and cpcG1 genes in Synechocystis on growth, pigmentation, accumulation of a stress-related peroxiredoxin protein, and reactive oxygen species (ROS) production
CpcG1 connects rods to the core and deletion of the cpcG1 gene is expected to yield cells containing only AP cores on the thylakoid membrane or destabilized cores (Kondo et al, 2005)
Summary
Phycobilisomes (PBSs) are abundant light-harvesting protein complexes, which comprise up to 60% of the total protein content of cyanobacteria (Singh et al, 2015). The reduced accumulation of OCP in the cpcF mutant of F. diplosiphon, which was observed under both continuous and fluctuating light conditions, implies that there may be potential feedback from cellular PBS levels to regulate OCP abundance (Agostoni et al, 2016). To assess whether the reduced OCP accumulation phenotype observed for a cpcF mutant in F. diplosiphon is observed for other cyanobacterial strains and to test the potential fitness implications of the interaction between CpcG1 and OCP, we assessed PBS-deficient strains of Synechocystis. We measured fitness as relative growth of the mutant strains compared to WT and assessed oxidative stress responses These studies identified cellular stress responses associated with reduced PBS abundance, including identifying a peroxiredoxin as a downstream effector of CpcF-mediated oxidative stress responses
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