The evolutionary development of the protein complement of Photosystem 2

During the transition from anoxygenic to oxygenic photosynthesis, the Type 2 reaction center underwent many changes, none so dramatic as the remarkable increase in complexity at the protein level, from only three or four subunits in the anoxygenic reaction center to possibly more than 25 in Photosystem 2 (PS2). The evolutionary source of most of these proteins is enigmatic, as they have no apparent homology to any other proteins in existing databases. However, some of the proteins in PS2 have apparent homologies to each other, suggesting ancient gene duplications have played an important role in the development of the complex. These homologies include the well-known examples of the D1 and D2 reaction center core proteins and the CP43 and CP47 core antenna proteins. In addition, PsbE and PsbF, the two subunits comprising cytochrome b-559, show homology to each other, suggesting that a homodimeric cytochrome preceded the heterodimeric one. Other potential homologies that appear to be statistically significant include PsbV with the N-terminal part of D1 and PsbT with PsbI. Most of the proteins that make up the photosynthetic apparatus bear no relation to any other proteins from any source. This suggests that a period of remarkable evolutionary innovation took place when the ability to make oxygen was invented. This was probably a response to the production of highly toxic oxygen and these new proteins served to protect and repair the photosynthetic apparatus from the harmful effects of oxygen.