Integration of Apo-α-Phycocyanin into Phycobilisomes and Its Association with FNR_L in the Absence of the Phycocyanin α-Subunit Lyase (CpcF) in <i>Synechocystis</i> sp. PCC 6803

<div><p>Phycocyanin is an important component of the phycobilisome, which is the principal light-harvesting complex in cyanobacteria. The covalent attachment of the phycocyanobilin chromophore to phycocyanin is catalyzed by the enzyme phycocyanin lyase. The photosynthetic properties and phycobilisome assembly state were characterized in wild type and two mutants which lack holo-α-phycocyanin. Insertional inactivation of the phycocyanin α-subunit lyase (Δ<i>cpcF</i> mutant) prevents the ligation of phycocyanobilin to α-phycocyanin (CpcA), while disruption of the <i>cpcB/A/C2/C1</i> operon in the CK mutant prevents synthesis of both apo-α-phycocyanin (apo-CpcA) and apo-β-phycocyanin (apo-CpcB). Both mutants exhibited similar light saturation curves under white actinic light illumination conditions, indicating the phycobilisomes in the Δ<i>cpcF</i> mutant are not fully functional in excitation energy transfer. Under red actinic light illumination, wild type and both phycocyanin mutant strains exhibited similar light saturation characteristics. This indicates that all three strains contain functional allophycocyanin cores associated with their phycobilisomes. Analysis of the phycobilisome content of these strains indicated that, as expected, wild type exhibited normal phycobilisome assembly and the CK mutant assembled only the allophycocyanin core. However, the Δ<i>cpcF</i> mutant assembled phycobilisomes which, while much larger than the allophycocyanin core observed in the CK mutant, were significantly smaller than phycobilisomes observed in wild type. Interestingly, the phycobilisomes from the Δ<i>cpcF</i> mutant contained holo-CpcB and apo-CpcA. Additionally, we found that the large form of FNR (FNR_L) accumulated to normal levels in wild type and the Δ<i>cpcF</i> mutant. In the CK mutant, however, significantly less FNR_L accumulated. FNR_L has been reported to associate with the phycocyanin rods in phycobilisomes via its N-terminal domain, which shares sequence homology with a phycocyanin linker polypeptide. We suggest that the assembly of apo-CpcA in the phycobilisomes of Δ<i>cpcF</i> can stabilize FNR_L and modulate its function. These phycobilisomes, however, inefficiently transfer excitation energy to Photosystem II.</p>