Diversification of Ubiquinone Biosynthesis via Gene Duplications, Transfers, Losses, and Parallel Evolution.

The availability of an ever-increasing diversity of prokaryotic genomes and metagenomes represents a major opportunity to understand and decipher the mechanisms behind the functional diversification of microbial biosynthetic pathways. However, it remains unclear to what extent a pathway producing a specific molecule from a specific precursor can diversify. In this study, we focus on the biosynthesis of ubiquinone (UQ), a crucial coenzyme that is central to the bioenergetics and to the functioning of a wide variety of enzymes in Eukarya and Pseudomonadota (a subgroup of the formerly named Proteobacteria). UQ biosynthesis involves three hydroxylation reactions on contiguous carbon atoms. We and others have previously shown that these reactions are catalyzed by different sets of UQ-hydroxylases that belong either to the iron-dependent Coq7 family or to the more widespread flavin monooxygenase (FMO) family. Here, we combine an experimental approach with comparative genomics and phylogenetics to reveal how UQ-hydroxylases evolved different selectivities within the constrained framework of the UQ pathway. It is shown that the UQ-FMOs diversified via at least three duplication events associated with two cases of neofunctionalization and one case of subfunctionalization, leading to six subfamilies with distinct hydroxylation selectivity. We also demonstrate multiple transfers of the UbiM enzyme and the convergent evolution of UQ-FMOs toward the same function, which resulted in two independent losses of the Coq7 ancestral enzyme. Diversification of this crucial biosynthetic pathway has therefore occurred via a combination of parallel evolution, gene duplications, transfers, and losses.

The Biosynthetic Pathway of Ubiquinone Contributes to Pathogenicity of Francisella novicida.

Francisella tularensis is the causative agent of tularemia. Because of its extreme infectivity and high mortality rate, this pathogen was classified as a biothreat agent. Francisella spp. are strict aerobes, and ubiquinone (UQ) has been previously identified in these bacteria. While the UQ biosynthetic pathways were extensively studied in Escherichia coli, allowing the identification of 15 Ubi proteins to date, little is known about Francisella spp. In this study, and using Francisella novicida as a surrogate organism, we first identified ubiquinone 8 (UQ8) as the major quinone found in the membranes of this bacterium. Next, we characterized the UQ biosynthetic pathway in F. novicida using a combination of bioinformatics, genetics, and biochemical approaches. Our analysis disclosed the presence in Francisella of 10 putative Ubi proteins, and we confirmed 8 of them by heterologous complementation in E. coli. The UQ biosynthetic pathways from F. novicida and E. coli share similar patterns. However, differences were highlighted: the decarboxylase remains unidentified in Francisella spp., and homologs of the Ubi proteins involved in the O2-independent UQ pathway are not present. This is in agreement with the strictly aerobic niche of this bacterium. Next, via two approaches, i.e., the use of an inhibitor (3-amino-4-hydroxybenzoic acid) and a transposon mutant, both of which strongly impair the synthesis of UQ, we demonstrated that UQ is essential for the growth of F. novicida in respiratory medium and contributes to its pathogenicity in Galleria mellonella used as an alternative animal model. IMPORTANCE Francisella tularensis is the causative bacterium of tularemia and is classified as a biothreat agent. Using multidisciplinary approaches, we investigated the ubiquinone (UQ) biosynthetic pathway that operates in F. novicida used as a surrogate. We show that UQ8 is the major quinone identified in the membranes of Francisella novicida. We identified a new competitive inhibitor that strongly decreased the biosynthesis of UQ. Our demonstration of the crucial roles of UQ for the respiratory metabolism of F. novicida and for the involvement in its pathogenicity in the Galleria mellonella model should stimulate the search for selective inhibitors of bacterial UQ biosynthesis.

Advances in bacterial pathways for the biosynthesis of ubiquinone.

Ubiquinone is an important component of the electron transfer chains in proteobacteria and eukaryotes. The biosynthesis of ubiquinone requires multiple steps, most of which are common to bacteria and eukaryotes. Whereas the enzymes of the mitochondrial pathway that produces ubiquinone are highly similar across eukaryotes, recent results point to a rather high diversity of pathways in bacteria. This review focuses on ubiquinone in bacteria, highlighting newly discovered functions and detailing the proteins that are known to participate to its biosynthetic pathways. Novel results showing that ubiquinone can be produced by a pathway independent of dioxygen suggest that ubiquinone may participate to anaerobiosis, in addition to its well-established role for aerobiosis. We also discuss the supramolecular organization of ubiquinone biosynthesis proteins and we summarize the current understanding of the evolution of the ubiquinone pathways relative to those of other isoprenoid quinones like menaquinone and plastoquinone.

A Soluble Metabolon Synthesizes the Isoprenoid Lipid Ubiquinone.

Ubiquinone (UQ) is a polyprenylated lipid that is conserved from bacteria to humans and is crucial to cellular respiration. How the cell orchestrates the efficient synthesis of UQ, which involves the modification of extremely hydrophobic substrates by multiple sequential enzymes, remains an unresolved issue. Here, we demonstrate that seven Ubi proteins form the Ubi complex, a stable metabolon that catalyzes the last six reactions of the UQ biosynthetic pathway in Escherichia coli. The SCP2 domain of UbiJ forms an extended hydrophobic cavity that binds UQ intermediates inside the 1-MDa Ubi complex. We purify the Ubi complex from cytoplasmic extracts and demonstrate that UQ biosynthesis occurs in this fraction, challenging the current thinking of a membrane-associated biosynthetic process. Collectively, our results document a rare case of stable metabolon and highlight how the supramolecular organization of soluble enzymes allows the modification of hydrophobic substrates in a hydrophilic environment.