Research Article

Light-dependent chlorophyll f synthase is a highly divergent paralog of PsbA of photosystem II

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Science  26 Aug 2016:
Vol. 353, Issue 6302, aaf9178
DOI: 10.1126/science.aaf9178

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Sometimes, red light means grow

Some cyanobacteria are able to use the far-red end of the light spectrum by synthesizing chlorophyll f pigments. Introducing the protein responsible for chlorophyll f synthesis into crop plants could potentially expand the range of wavelengths that such plants use during photosynthesis and thereby increase their growth efficiency. Ho et al. identified chlorophyll f synthase (ChlF) in two cyanobacteria that are acclimatized to grow using far-red light. Introducing the ChlF-encoding gene into a model cyanobacterium allowed the organism to synthesize chlorophyll f. Similarities between ChlF and a core protein of photosystem II suggest that they have a close evolutionary relationship, and ChlF may even represent a more primitive photochemical reaction center.

Science, this issue p. 886

Structured Abstract

INTRODUCTION

Terrestrial cyanobacteria often occur in environments that receive strongly filtered light because of shading by plants or because of their associations with soil crusts, benthic mat communities, or dense cyanobacterial blooms. The light in such environments becomes highly enriched in far-red light (FRL) (wavelengths >700 nm). Cyanobacteria that are able to use FRL for photosynthesis have evolved a novel far-red light photoacclimation (FaRLiP) mechanism to gain a strong selective advantage over other cyanobacteria. The FaRLiP response involves extensive remodeling of photosystems I and II (PSI and PSII) and light-harvesting phycobilisome complexes. FaRLiP cells synthesize chlorophyll f (Chl f), Chl d, and FRL-absorbing phycobiliproteins under these conditions and thus can use FRL efficiently for oxygenic photosynthesis. A key element of the FaRLiP response is the FRL-specific expression of 17 genes that encode paralogs of core components of the three light-harvesting complexes produced during growth in white light.

RATIONALE

The ability to synthesize Chl f is a key element of the FaRLiP response, but the Chl f synthase had remained unknown. Transcription and phylogenetic profiling suggested that the gene(s) responsible for this activity were in the conserved FaRLiP gene cluster. This led us to focus on psbA4, a divergent member of the psbA gene family encoding so-called “super-rogue” PsbA, a paralog to the D1 core subunit of PSII. We used reverse genetics and heterologous expression to identify the Chl f synthase of two cyanobacteria capable of FaRLiP: Chlorogloeopsis fritschii PCC 9212 and Synechococcus sp. PCC 7335.

RESULTS

In both species, null mutants of psbA4 no longer synthesized Chl f and lacked FRL absorption and long-wavelength fluorescence emission, the key spectroscopic properties associated with Chl f. Heterologous expression of the psbA4 gene from C. fritschii PCC 9212 in the model non-FaRLiP cyanobacterium Synechococcus sp. PCC 7002 led to the synthesis of Chl f. These results showed that psbA4 (renamed chlF) encodes the Chl f synthase. Growth experiments using intervals of FRL and darkness showed that Chl f synthesis is light-dependent, which implies that ChlF is a photo-oxidoreductase that oxidizes Chl a (or Chlide a) instead of water.

CONCLUSION

ChlF may have evolved after gene duplication from PsbA of a water-oxidizing PSII complex by loss of the ligands for binding the Mn4Ca1O5 cluster but by retaining catalytically useful chlorophylls, tyrosine YZ, and plastoquinone binding. Alternatively, PsbA may have arisen by gene duplication from ChlF and then by gaining the capacity to bind the Mn4Ca1O5 cluster. Because ChlF seems likely to function as a simple homodimer and belongs to the earliest diverging clade of PsbA sequences in phylogenetic analyses, Chl f synthase may have been the antecedent of water-oxidizing PSII. This hypothesis provides a simple explanation for the occurrence of multiple reaction centers in an ancestral cyanobacterial cell. Thus, a Chl a photo-oxidoreductase that initially evolved for enhanced use of FRL may explain the origin of oxygen-evolving PSII. From an applied perspective, knowing the identity of ChlF may provide a tractable route for introducing the capacity for FRL use into crop plants, greatly expanding the wavelength range that they can use to conduct photosynthesis.

Structural model of ChlF homodimer based on PsbA of PSII.

The PsbA4/ChlF homodimer polypeptides are shown as blue ribbon structures. Putative substrate chlorophyll (Chl) a molecules are shown in bright green at the bottom. Cofactors are Chl a (dark green), pheophytin a (pale green), plastoquinones (burgundy), β-carotenes(yellow), and tyrosine YZ (magenta).

Abstract

Chlorophyll f (Chl f) permits some cyanobacteria to expand the spectral range for photosynthesis by absorbing far-red light. We used reverse genetics and heterologous expression to identify the enzyme for Chl f synthesis. Null mutants of “super-rogue” psbA4 genes, divergent paralogs of psbA genes encoding the D1 core subunit of photosystem II, abolished Chl f synthesis in two cyanobacteria that grow in far-red light. Heterologous expression of the psbA4 gene, which we rename chlF, enables Chl f biosynthesis in Synechococcus sp. PCC 7002. Because the reaction requires light, Chl f synthase is probably a photo-oxidoreductase that employs catalytically useful Chl a molecules, tyrosine YZ, and plastoquinone (as does photosystem II) but lacks a Mn4Ca1O5 cluster. Introduction of Chl f biosynthesis into crop plants could expand their ability to use solar energy.

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