Figure 8.

Model of Evolution of the PSII Extrinsic Protein Domain.

(A) Ancestral PSII with four subunits in the extrinsic domain. In this model, G. violaceus is the modern representation of such an ancestral organism. PsbQ was acquired before the split separating the cyanobacteria (Synechocystis) from the eukaryotic photosynthetic organisms. PsbV was lost before the separation of red algae (C. merolae) and green photosynthetic eukaryotes (Arabidopsis), and PsbU was lost only in the plant lineage. O, PsbO; P, PsbP; Q, PsbQ; U, PsbU; V, PsbV.

(B) Ancestral PSII with five subunits in the extrinsic domain. In this model, the nonthylakoid cyanobacterial lineage lost PsbQ (G. violaceus), and other cyanobacteria (Synechocystis) represent the ancestral organism. As described above, PsbV was lost before the red algae (C. merolae) separation, and PsbU was lost after this separation from green photosynthetic eukaryotes (Arabidopsis).