The evolution of strictly monofunctional naphthoquinol C-methyltransferases is vital in cyanobacteria and plastids.

Prenylated quinones are membrane-associated metabolites that serve as vital electron carriers for respiration and photosynthesis. The UbiE (EC 2.1.1.201)/MenG (EC 2.1.1.163) C-methyltransferases catalyze pivotal ring methylations in the biosynthetic pathways of many of these quinones. In a puzzling evolutionary pattern, prokaryotic and eukaryotic UbiE/MenG homologs segregate into two clades. Clade 1 members occur universally in prokaryotes and eukaryotes, excluding cyanobacteria, and include mitochondrial COQ5 enzymes required for ubiquinone biosynthesis; clade 2 members are specific to cyanobacteria and plastids. Functional complementation of an Escherichia coli ubiE/menG mutant indicated that clade 1 members display activity with both demethyl-benzoquinols and demethyl-naphthoquinols, independently of the quinone profile of their original taxa, while clade 2 members have evolved strict substrate specificity for demethyl-naphthoquinols. Expression of the gene encoding bi-functional Arabidopsis (Arabidopsis thaliana) COQ5 in the cyanobacterium Synechocystis or its retargeting to Arabidopsis plastids resulted in synthesis of a methylated variant of plastoquinone-9 that does not occur in nature. Accumulation of methyl-plastoquinone-9 was acutely cytotoxic, leading to the emergence of suppressor mutations in Synechocystis and seedling lethality in Arabidopsis. These data demonstrate that in cyanobacteria and plastids, co-occurrence of phylloquinone and plastoquinone-9 have driven the evolution of mono-functional demethyl-naphthoquinol methyltransferases, and explain why plants cannot capture the intrinsic bi-functionality of UbiE/MenG to simultaneously synthesize their respiratory and photosynthetic quinones.