Characterization of red-shifted phycobilisomes isolated from the chlorophyll f -containing cyanobacterium Halomicronema hongdechloris

Abstract

Phycobilisomes are the main light-harvesting protein complexes in cyanobacteria and some algae. It is commonly accepted that these complexes only absorb green and orange light, complementing chlorophyll absorbance. Here, we present a new phycobilisome derived complex that consists only of allophycocyanin core subunits, having red-shifted absorption peaks of 653 and 712 nm. These red-shifted phycobiliprotein complexes were isolated from the chlorophyll f-containing cyanobacterium, Halomicronema hongdechloris, grown under monochromatic 730 nm-wavelength (far-red) light. The 3D model obtained from single particle analysis reveals a double disk assembly of 120-145 Å with two α/β allophycocyanin trimers fitting into the two separated disks. They are significantly smaller than typical phycobilisomes formed from allophycocyanin subunits and core-membrane linker proteins, which fit well with a reduced distance between thylakoid membranes observed from cells grown under far-red light. Spectral analysis of the dissociated and denatured phycobiliprotein complexes grown under both these light conditions shows that the same bilin chromophore, phycocyanobilin, is exclusively used. Our findings show that red-shifted phycobilisomes are required for assisting efficient far-red light harvesting. Their discovery provides new insights into the molecular mechanisms of light harvesting under extreme conditions for photosynthesis, as well as the strategies involved in flexible chromatic acclimation to diverse light conditions.

Light-dependent chlorophyll f synthase is a highly divergent paralog of PsbA of photosystem II

“…The expression of the paralogous genes in FRL leads to extensive remodeling of the cores of the major light-absorbing complexes of cyanobacteria: phycobilisomes (9), photosystem I (PSI) (10), and PSII (6)(7)(8)11). About 10% of the chlorophyll a (Chl a) molecules are replaced by Chl f in the paralogous PSI and PSII complexes produced in FRL (7,8), and allophycocyanin B variants absorbing beyond 700 nm are also synthesized during FaRLiP (7,8,12,13). Together, these changes allow cells to make use of FRL for growth (7,8).…”

Cyanobacterial Production of Biopharmaceutical and Biotherapeutic Proteins

“…SDS-PAGE was incubated in 5 mM zinc sulfate for 30 min (Li et al, 2016). To detect covalent chromophore-binding polypeptides, zinc induced fluorescence was monitored by Chemidoc imaging system (BIORAD), employing UV light as a light source.…”

Chromatic Acclimation in Cyanobacteria: A Diverse and Widespread Process for Optimizing Photosynthesis

“…The gene cluster implicated in conferring CA6/FaRLiP is conserved in thirteen cyanobacterial species (36). A CA6/FaRLiP-like gene cluster is also present in the cyanobacterium Halomicronema hongdechloris, which produces chlorophylls a and f but not chlorophyll d. After a shift to far-red light, this species loses its PBS rods and remodels the PBS core to contain variants of allophycocyanin that have red-shifted absorption characteristics, matching the absorption features of the chlorophyll f produced in far-red light (70). Also, after shifts from white to far-red light, the chlorophyll composition of photosystem I changes from all chlorophyll a to 92% chlorophyll a and approximately 8% chlorophyll f (69).…”