Abstract
BackgroundPsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II. It plays a crucial role in the stabilisation of the water-splitting Mn4CaO5 cluster, which catalyses the oxidation of water to molecular oxygen by using light energy. PsbO was also demonstrated to have a weak GTPase activity that could be involved in regulation of D1 protein turnover. Our analysis of psbO sequences showed that many angiosperm species express two psbO paralogs, but the pairs of isoforms in one species were not orthologous to pairs of isoforms in distant species.ResultsPhylogenetic analysis of 91 psbO sequences from 49 land plant species revealed that psbO duplication occurred many times independently, generally at the roots of modern angiosperm families. In spite of this, the level of isoform divergence was similar in different species. Moreover, mapping of the differences on the protein tertiary structure showed that the isoforms in individual species differ from each other on similar positions, mostly on the luminally exposed end of the β-barrel structure. Comparison of these differences with the location of differences between PsbOs from diverse angiosperm families indicated various selection pressures in PsbO evolution and potential interaction surfaces on the PsbO structure.ConclusionsThe analyses suggest that similar subfunctionalisation of PsbO isoforms occurred parallelly in various lineages. We speculate that the presence of two PsbO isoforms helps the plants to finely adjust the photosynthetic apparatus in response to variable conditions. This might be mediated by diverse GTPase activity, since the isoform differences predominate near the predicted GTP-binding site.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0523-4) contains supplementary material, which is available to authorized users.
Highlights
PsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II
In A. thaliana and Populus trichocarpa we found that chromosomal segments containing psbO paralogs are collinear, which suggests that psbO was duplicated within the context of a larger-scale duplication
Our study showed that the pairs of PsbO isoforms evolved in numerous angiosperm lineages independently
Summary
PsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II. It plays a crucial role in the stabilisation of the water-splitting Mn4CaO5 cluster, which catalyses the oxidation of water to molecular oxygen by using light energy. Photosynthetic conversion of light into chemical energy in oxygenic phototrophs is accompanied with evolution of molecular oxygen released from water molecules. This process is realized in the oxygen evolving complex of photosystem II present in thylakoid membranes. Photosystem II (PSII) is a multisubunit protein–cofactor complex that uses light energy to oxidize water and to reduce. Relatively high pairwise identity between PsbO sequences of Thermosynechococcus and higher plants (around 45 %) allows construction of homologous models for plant PsbOs [9, 10]
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