cpcB

cpcB is the gene that encodes the beta subunit of phycocyanin, a phycobiliprotein that forms part of the light-harvesting phycobilisome in many cyanobacteria and some red algae. The beta subunit pairs with the alpha subunit (encoded by cpcA) to assemble hexameric rods that attach to the core of the phycobilisome. Phycocyanin absorbs light most efficiently in the orange to red region of the spectrum (approximately 620–650 nm) and transfers excitation energy to the photosystems, primarily photosystem II, via the phycobilisome complex.

Genetically, cpcB is typically co-transcribed with cpcA in an operon (cpcBA) and encodes a polypeptide of roughly 150–170 amino acids. The encoded protein contains conserved cysteine residues that covalently bind the phycocyanobilin chromophore, giving phycocyanin its characteristic color. The beta subunit contributes to the spectral properties and the assembly of the phycobilisome together with linker proteins such as CpcC and CpcD that organize rod structure.

Expression of cpcB is influenced by light conditions and nutrient status, and its regulation is coordinated with other phycobiliprotein genes to optimize light capture under different environmental conditions. Homologs of cpcB are found across cyanobacteria and in some algal lineages that retain phycobiliprotein-based light-harvesting systems.

Beyond its biological role, cpcB serves as a model in studies of photosynthetic light harvesting and has been explored in biotechnological contexts for engineering light-absorbing complexes.