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

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 PSIPsaJF. PSIPsaJF is promiscuous for its electron donors and can accept electrons from respiratory cytochromes. We solved the structure of PSIPsaJF 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

Structure of the plant photosystem I supercomplex at 2.6 Å resolution

“…A stacked chlorophyll trimer supported by an extended loop in PsaB was the best candidate for one of the strong red absorbers in this complex (Jordan et al, 2001). PsaB sequences from eukaryotes and many cyanobacteria lack this extended loop, resulting in this chlorophyll trimer being lost, as has been shown in the plant and mesophilic PSI structures (Amunts et al, 2010;Mazor et al, 2014). At the current resolution, we observed new core chlorophyll bound between PsaG and Lhca1 and a newly discovered lipid ( Figure 4A).…”

Closing the gaps on the viral photosystem‐I psaDCAB gene organization

“…It is important to note that no petE genes were found on any of the viral PSI metagenomic scaffolds retrieved in this study. Alternatively, the change in the viral PsaA loop may perhaps have a similar function as the fused PsaJF protein found in the viral psaJF→C→A→B→K→E→D cassette, namely being promiscuous for its electron donors and being able to accept electrons from electron donors other than plastocyanin or cytochrome c 6 (Mazor et al, 2014).…”

“…The fused PsaJF protein from the low %G + C cassette was hypothesized to be able to accept electrons not only from PSII (via plastocyanin or cytochrome c 6) but to also work with other electron donors like soluble cytochrome c that usually function as electron donors to cytochrome oxidase (Sharon et al, 2009). This was recently shown using a heterologous Synechocystis system mimicking the cyanophage system (Mazor et al, 2014).…”

Carotenoids affect the structure and functions of the Cyanobacterial photosynthetic complexes

“…that is presumed to support the trimerization (Fromme et al 2001). Partial structure of PSI monomer of Synechocystis has been also obtained and revealed some structural differences between the two species (Mazor et al 2014), but further studies are needed to reveal the exact structure of trimeric PSI from Synechocystis.…”

“…Around 5-10% of the PSI associated Chls absorb light energy at longer wavelengths; these are the so-called red-shifted or long-wavelength Chls (LWCs). The LWCs were proposed to originate from Chl-Chl interactions induced by the aggregation of Chls within the PSI complex (Fromme et al 2001, Mazor et al 2014. Generally, the number of LWCs is higher in the PSI trimers than in monomers in the studied cyanobacterial species, but their amount and spectral properties vary among species (El-Mohsnawy et al 2010, .…”