Functional Modification of Cyanobacterial Phycobiliprotein and Phycobilisomes through Bilin Metabolism Control

Abstract

The light-harvesting antenna complexes in cyanobacteria are known as phycobilisomes (PBSs), and they can adapt to a diverse range of light environments owing to the deployment of different phycobiliproteins within the PBS structures. Freshwater cyanobacteria such as Synechococcus elongatus PCC 7942 thrive under red light due to phycocyanin (PC), along with phycocyanobilin (PCB), in PBS. Conversely, cyanobacteria in shorter-wavelength light environments, such as green light, employ phycoerythrin paired with phycoerythrobilin (PEB) alongside PC in PBS. Synthetic biology studies have shown that PEB production can be enhanced by additional expression of the heterologous PEB synthases, namely 15,16-dihydrobiliverdin:ferredoxin oxidoreductase and PEB:ferredoxin oxidoreductase (PebAB), leading to cellular browning due to PEB accumulation. This approach is genetically unstable, and the properties of the resulting PEB-bound PBS complexes remain uncharacterized. Herein, we engineered a novel strain of Synechococcus 7942 PEB1, with finely tuned control of PEB metabolism. PEB1 exhibited a remarkable and reversible color shift from green to brown and pink, based on the PebAB induction levels. High induction led to a complete PCB-to-PEB substitution, causing the disassembly of the PBS rod complex. In contrast, low PebAB levels resulted in the formation of a stable chimeric PBS complex with partial PCB-to-PEB substitution. This adaptation enabled efficient light harvesting in the green spectrum and energy transfer to the photosynthetic reaction center. These findings, which improve our understanding of PBS and highlight the structural importance of the bilin composition, lay the foundation for future PBS adaptation studies in bioengineering, synthetic biology, and renewable energy.