Partial photoswitching of rod-shaped phycobilisome production in the cyanobacterium <i>Synechocystis</i> sp. PCC 6803

Certain cyanobacteria can alter the structure of their photosynthetic supercomplex phycobilisome (PBS) in response to changes in ambient light color. This process is known as chromatic acclimation (CA) and is classified into seven types (CA1-CA7), each involving distinct modifications of PBS structure. Among them, CA1 is defined as the regulation of the rod-shaped PBS in response to green and red light, maintaining the excitation balance between the two photosystems. Synechocystis sp. PCC 6803 (PCC 6803) is a widely used model cyanobacterium for photosynthesis research and harbors the CcaSR photosensory gene cluster for CA1. In this study, we investigated the wavelength dependence of the rod-shaped PBS production in PCC 6803. RNA-Seq analysis revealed that the expression of cpcL, which encodes a rod-membrane linker protein of rod-shaped PBS, is upregulated under a wide range of visible light (470-630 nm) and partially suppressed under violet (380-420 nm) and red to far-red light (680-720 nm) conditions. Low-temperature fluorescence emission spectra revealed that the ratio of rod-shaped PBS to hemi-discoidal PBS was highest under green light (530 nm), followed by red (660 nm) and far-red (700 nm) conditions. Furthermore, the isolation of intact PBS fractions using an improved procedure revealed the presence of rod-shaped PBS containing CpcL under these light conditions. The incomplete photoswitching of the rod-shaped PBS production may be due to the adaptation of the photosensory CcaSR system in PCC 6803, which lacks green-absorbing components in its PBSs and provides green-light rich environments in their cell aggregates.