Characterization of Site-Directed Mutants in the Cytochrome c-550 Protein of Photosystem II

Photosynthesis is the process by which cyanobacteria, algae, and higher plants convert light energy to chemical energy via the biosynthesis of carbohydrates. Photosystem II is a multi-protein/pigment complex embedded in the thylakoid membrane. CP43 is the product of the psbC gene, and is an intrinsic component of the photosystem II complex. CP43 is 473 amino acids long and has five hydrophilic loops connecting six transmembrane alpha helices, including a large extrinsic loop E which is exposed to the lumenal side of the thylakoid membrane. Short deletions of regions of the loop E of CP43 resulted in mutants that fail to grow photoautotrophically and are devoid of oxygen-evolving activity. A mutation was introduced in loop E that altered arginine at position 305 to a serine residue, producing a mutant with severely reduced growth and oxygen evolving activity under chloride limiting conditions. However, when grown under normal conditions, the R305S mutant showed no extreme phenotype. After isolation of photosystem II particles and chemilumininescent staining for cytochromes, it was determined that this specific mutation resulted in loss of binding of an extrinsic photosystem II protein, cytochrome c-550. Deletion of the psbV gene encoding cytochrome c-550 resulted in the loss of photoautotrophic growth in media lacking chloride and/or calcium. X-ra