Phycobilisomes (PBSs) are accessory light harvesting antennae that bind to the thylakoid membranes in cyanobacteria and help shuttle light energy to the photosystems for photosynthesis. These large protein complexes are modified during a process called chromatic acclimation (CA). During CA, cyanobacteria perceive and respond to changes in their light environment to maximize productivity and minimize damage caused by excess light. During a routine color mutant screening, a blue-green mutant was found and determined to have a transposon within the phycobilisome abundance regulator gene, psoR, in Fremyella diplosiphon. The lack of PsoR disrupted the organism’s ability to change pigmentation/PBSs under different light conditions. Along with phenotypic characterization, bioinformatic studies were performed to uncover the putative function of psoR, and phylogenetic analysis conducted to determine if this gene was found in all PBS-containing organisms or unique to certain species. PsoR was found to have homology to β-CASP proteins, which are involved in nucleic acid processing, which suggests PsoR may be involved in regulating CA-related genes. The resulting phylogenetic tree showed homologs present throughout the cyanobacteria phylum, but noticeably absent in Gloeobacter, an ancient clade of cyanobacteria that lacks thylakoid membranes and has atypical PBSs. Overall, these results suggest that PsoR is highly conserved amongst cyanobacteria and could play a role in the regulation of PBSs and CA in cyanobacteria.
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