The Biogenesis of the Thylakoid Membrane: Photosystem II, a Case Study

Abstract

The thylakoid membrane of cyanobacteria and higher plants is a highly organized system of internal membranes that enclose the protein complexes of the photosynthetic apparatus. Photosystem II (PSII) is a central component of this machinery and it is responsible for catalyzing the light induced electron transfer from water to plastoquinone, generating almost all of the oxygen in the atmosphere. PSII is a multimeric chlorophyll-protein complex of dual genetic origin, whose biogenesis is a complicated and highly regulated process. Nuclear-encoded auxiliary factors coordinate the expression of plastid- and nuclear-encoded PSII subunits in response to environmental and endogenous cues. Studies performed with photosynthesis mutants of green algae and higher plants have revealed that these factors control the expression of the plastid-encoded genes at the posttranscriptional level. Many of the regulatory proteins are able to modify the efficiency of translation of specific PSII subunit transcripts, and redox regulation plays an important role in the control of these processes. The assembly of PSII proteins into a functional complex also requires the activity of nuclear-encoded factors, several of which are conserved throughout the plant kingdom, but some appear only in higher plants and green algae, indicating that new requirements for PSII assembly have evolved during evolution. Recent studies suggest that the initial steps of PSII biogenesis are localized to a specific region of the interior membrane system of chloroplasts and cyanobacteria.