The Structure, Function and Biogenesis of Cytochrome b6f Complexes

Abstract

In most photosynthetic prokaryotes, cytochrome-containing complexes—cytochrome bc1 and cytochrome b6f— are also part of respiratory chains, whereas the photochemical reaction centers and their light-harvesting antennae function only in photosynthesis. Thus, genetic approaches, which are intimately associated with the possibility to inactivate and restore a function, have met with limited success in the case of cytochromecontaining complexes because their inactivation is usually lethal. This is in great contrast with the situation in Chlamydomonas reinhardtii which shows dispensable photosynthesis and can be grown at the expense of exogeneous reduced carbon sources, utilizing a mitochondrial-based and cytochrome bc1-borne, coupled-electron flow. Thus, Chlamydomonas is the only photosynthetic organism which has been extensively used in cytochrome b6f studies. Owing to the genetics of Chlamydomonas, a number of cytochrome b6f mutants were isolated both by classical and transformation mutagenesis. These strains offered a unique opportunity to advance the understanding of the structure, biogenesis and function of cytochrome b6f in a photosynthetic eukaryote. This chapter reviews studies from the past decade, which combine mutational approaches with recent progress in membrane protein biochemistry and time-resolved flash spectroscopy. These studies have brought to light both the pivotal role of cytochrome b6f in photosynthesis and its value as a model system for general issues in the field of organellar gene expression and protein assembly.