Photosystem II Assembly from Scratch

Construction of a functional Photosystem II (PSII) in cyanobacteria and chloroplasts depends on the action of auxiliary factors, which transiently interact with PSII intermediates during assembly. In addition to a common PSII structure and a conserved set of PSII assembly factors, cyanobacteria, and higher plants have evolved additional, clade-specific assembly factors. Most such factors in cyanobacteria and chloroplasts have been identified by " top-down " approaches (forward and reverse genetics), which involved genetic disruption of individual components in the assembly process and subsequent characterization of the ensuing phenotypic effects on the respective mutant lines/strains. In contrast, a " bottom-up " strategy, based on the engineering of a synthetic bacterium with a plant-type PSII, has the potential to identify all assembly factors sufficient to make a functional plant PSII. Photosystem II (PSII) is a water-plastoquinone photo-oxidoreductase, which is found in cyanobacteria and their endosymbiotic descendants, the chloroplasts. Light-driven water splitting and subsequent electron transfer steps are carried out with the assistance of non-proteinaceous cofactors. Thus, the PSII monomer harbors a Mn 4 CaO 5 cluster, chloride, bicarbonate, 1-2 hemes, 1 nonheme iron, 35 chlorophyll a molecules, 2 pheophytins, 11 β-carotenes, and 2 plastoquinones (Umena et al., 2011), all of which are embedded in a shell made up of at least 20 proteins (Shen, 2015) that determine their correct positioning and relative orientation. Several PSII-associated lipids have been identified in crystal structures and might also be important for functionality (Mizusawa and Wada, 2012; Kansy et al., 2014). The structural core of PSII is conserved between chloroplasts and cyanobacteria (Allen et al., 2011). However, the oxygen-evolving complex in cyanobacteria contains subunits U and V, which are replaced by Q, R, P, and Tn in higher plants (Bricker et al., 2012). Furthermore, in contrast to the soluble, peripherally attached phycobilisomes found in cyanobacteria, green photosynthetic eukaryotes have evolved integrated light-harvesting complexes and lack phycobilisomes (Hohmann-Marriott and Blankenship, 2011; Figure 1A). In accordance with its structural complexity, the assembly of PSII is an elaborate and highly coordinated process, which depends on the action of a network of assembly factors and the fabrication of distinct metastable modules during the course of assembly (see for reviews on this topic: Nixon et al., 2010; Nickelsen and Rengstl, 2013; and papers in this special issue of Frontiers in Plant Science). Several modules common to cyanobacterial and chloroplast PSII assembly processes have been described and are characterized by transient binding …