Comparative Study of Phycoerythrobilin Synthases for Fine-Tuning Photosynthetic Light-Harvesting Complexes, Phycobilisomes
Phycobilisomes serve as the major light-harvesting antenna complexes in cyanobacteria; they capture light and transfer excitation energy to the photosystems, and their spectral and functional properties are primarily determined by the types of bilin chromophores attached to phycobiliproteins. We previously revealed that the properties of phycobilisomes can be modulated by altering the biosynthesis of these bilin chromophores. In this study, we introduced two distinct phycoerythrobilin (PEB) biosynthetic enzymes, pcyX and pebS, into cyanobacterium Synechococcus elongatus PCC 7942 and successfully enabled PEB incorporation into the native phycobilisome. PebS exhibited high PEB synthesis activity comparable to the canonical PebA-PebB pathway, whereas PcyX showed lower activity, enabling fine-tuning of PEB accumulation. Consistent with previous findings, moderate PEB supply enhanced growth under green light; notably, PcyX expression further promoted growth by providing a more balanced pigment supply. Transcriptome analysis confirmed comparable expression of pebA-pebB, pcyX, and pebS, and revealed that increased PEB accumulation triggered broad metabolic remodeling: genes for glucose metabolism, hydrogenase complexes, potassium transporters, and regulatory factors including sigma factors and circadian clock genes were upregulated, suggesting a shift toward dissipation of excess reducing power. These results indicate that natural diversity among bilin synthases can be exploited to engineer phycobilisome composition and redox homeostasis in cyanobacteria.