Abstract
Menaquinone (vitamin K2) shuttles electrons between membrane-bound respiratory complexes under microaerophilic conditions. In photosynthetic eukaryotes and cyanobacteria, phylloquinone (vitamin K1) participates in photosystem I function. Here we elucidate the evolutionary history of vitamin K metabolism in algae and plants. We show that Chlamydiales intracellular pathogens made major genetic contributions to the synthesis of the naphthoyl ring core and the isoprenoid side-chain of these quinones. Production of the core in extremophilic red algae is under control of a menaquinone (Men) gene cluster consisting of 7 genes that putatively originated via lateral gene transfer (LGT) from a chlamydial donor to the plastid genome. In other green and red algae, functionally related nuclear genes also originated via LGT from a non-cyanobacterial, albeit unidentified source. In addition, we show that 3–4 of the 9 required steps for synthesis of the isoprenoid side chains are under control of genes of chlamydial origin. These results are discussed in the light of the hypoxic response experienced by the cyanobacterial endosymbiont when it gained access to the eukaryotic cytosol.
Highlights
Membrane-bound polyunsaturated isoprenoid quinones (Fig. 1) undergo a two-step reversible reduction, making them ideal electron shuttles between different protein complexes, such as those involved in respiration and photosynthesis[1]
Phylloquinone instead of MK4 is synthesized in cyanobacteria and plants because geranylgeranyl diphosphate is reduced by GGPP reductase (GGPPR) prior to its transfer to the quinone core by the Men A prenyl transferase
Our analysis of the cluster provides evidence that the gene encoding (DHNA)-CoA thioesterase is physically linked to the Men genes in Cyanidiophytina. This expanded cluster defines a complete menaquinone synthesis pathway up to DHNA, with the exception of MenH that catalyzes the third step in the synthesis of the quinone core and is a component of the plant nuclear and mesophyllic rhodophycean phyllo fusions[4]
Summary
Membrane-bound polyunsaturated isoprenoid quinones (Fig. 1) undergo a two-step reversible reduction, making them ideal electron shuttles between different protein complexes, such as those involved in respiration and photosynthesis[1]. External provision of the widespread phylloquinone form of vitamin K requires substitution of the partially saturated isoprenoid phytyl side chain with polyunsaturated prenyl chains This reaction is carried out by UBIAD1, an essential prenyltranferase derived from the same family (MenA) as the bacterial gene responsible for the transfer of the isoprenoid polyunsaturated chain to the quinone core[3]. Are catalyzed in the plastid by a large nuclear-encoded protein fusion referred to as “PHYLLO” that produces o-succinyl benzoate, used in peroxisomes to yield the DHNA (1,4-dihydroxy-2-naphthoic acid) core of the quinone[4] This core is reimported into plastids, where it is prenylated by MenA, reduced by Ndc[1] (NAD(P)H dehydrogenase C), and methylated by MenG prior to assembly into PSI4,7. The timing and implications of these transfers are discussed in our paper
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