Crystal structures of virus-like photosystem I complexes from the mesophilic cyanobacterium Synechocystis PCC 6803

Yuval Mazor,Hila Toporik,Daniel Nataf,Nathan Nelson

doi:10.7554/elife.01496

Abstract

Oxygenic photosynthesis supports virtually all life forms on earth. Light energy is converted by two photosystems-photosystem I (PSI) and photosystem II (PSII). Globally, nearly 50% of photosynthesis takes place in the Ocean, where single cell cyanobacteria and algae reside together with their viruses. An operon encoding PSI was identified in cyanobacterial marine viruses. We generated a PSI that mimics the salient features of the viral complex, named PSI(PsaJF). PSI(PsaJF) is promiscuous for its electron donors and can accept electrons from respiratory cytochromes. We solved the structure of PSI(PsaJF) and a monomeric PSI, with subunit composition similar to the viral PSI, providing for the first time a detailed description of the reaction center and antenna system from mesophilic cyanobacteria, including red chlorophylls and cofactors of the electron transport chain. Our finding extends the understanding of PSI structure, function and evolution and suggests a unique function for the viral PSI. DOI: http://dx.doi.org/10.7554/eLife.01496.001.

Highlights

Oxygenic photosynthesis, which takes place in cyanobacteria, algae, and plants, provides most of the food and fuel on Earth (Barber, 2004; Nelson, 2011)
We currently know only two versions of the type I reaction centers: the relatively simple bacterial homodimer found in green sulfur bacteria (GSB), heliobacteria and Chloracidobacterium or the much more complex photosystem I (PSI) with its 11–15 subunits found in cyanobacteria and all photosynthetic eukaryotes (Buttner et al, 1992; Hauska et al, 2001; Nelson and Yocum, 2006)
We present a high resolution model for the Synechocystis PSI monomer, which allows us to identify for the first time the changes in the antenna system and electron transport chain (ETC) that exist between Synechocystis and Thermosynechococcus

Summary

Introduction

Oxygenic photosynthesis, which takes place in cyanobacteria, algae, and plants, provides most of the food and fuel on Earth (Barber, 2004; Nelson, 2011). We currently know only two versions of the type I reaction centers: the relatively simple bacterial homodimer found in green sulfur bacteria (GSB), heliobacteria and Chloracidobacterium or the much more complex PSI with its 11–15 subunits found in cyanobacteria and all photosynthetic eukaryotes (Buttner et al, 1992; Hauska et al, 2001; Nelson and Yocum, 2006). Sequence conservation between the PsaA/B subunits of PSI and the single large subunit of the homodimeric reaction centers is low, their inferred membrane topology is similar, and the sequence conservation around the bound iron sulfur cluster is high, supporting the notion of a common ancestor (Mulkidjanian and Junge, 1997; Blankenship and Hartman, 1998; Baymann et al, 2001; Mazor et al, 2012; Rutherford et al, 2012)

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