Abstract
Many proteobacteria, such as Escherichia coli, contain two main types of quinones: benzoquinones, represented by ubiquinone (UQ) and naphthoquinones, such as menaquinone (MK), and dimethyl-menaquinone (DMK). MK and DMK function predominantly in anaerobic respiratory chains, whereas UQ is the major electron carrier in the reduction of dioxygen. However, this division of labor is probably not very strict. Indeed, a pathway that produces UQ under anaerobic conditions in an UbiU-, UbiV-, and UbiT-dependent manner has been discovered recently in E. coli Its physiological relevance is not yet understood, because MK and DMK are also present in E. coli Here, we established that UQ9 is the major quinone of Pseudomonas aeruginosa and is required for growth under anaerobic respiration (i.e. denitrification). We demonstrate that the ORFs PA3911, PA3912, and PA3913, which are homologs of the E. coli ubiT, ubiV, and ubiU genes, respectively, are essential for UQ9 biosynthesis and, thus, for denitrification in P. aeruginosa These three genes here are called ubiTPa , ubiVPa , and ubiUPa We show that UbiVPa accommodates an iron-sulfur [4Fe-4S] cluster. Moreover, we report that UbiUPa and UbiTPa can bind UQ and that the isoprenoid tail of UQ is the structural determinant required for recognition by these two Ubi proteins. Since the denitrification metabolism of P. aeruginosa is believed to be important for the pathogenicity of this bacterium in individuals with cystic fibrosis, our results highlight that the O2-independent UQ biosynthetic pathway may represent a target for antibiotics development to manage P. aeruginosa infections.
Highlights
The opportunistic pathogen Pseudomonas aeruginosa has a remarkable ability to grow under a variety of environmental conditions, such as soil and water as well as animal, human, and plant-host-associated environments
The quinone content of P. aeruginosa PAO1, grown under ambient air or anaerobic conditions, was determined using electrochemical detection of lipid extracts separated by HPLC and compared with those obtained from E. coli
It is interesting that the UQ content of E. coli was higher under aerobic compared with anaerobic conditions (97 6 4 versus 42 6 6 pmol of UQ8 per mg of cells), whereas we found the opposite for P. aeruginosa (95 6 4 versus 126 6 4 pmol of ubiquinone 9 (UQ9) per mg of cells)
Summary
The opportunistic pathogen Pseudomonas aeruginosa has a remarkable ability to grow under a variety of environmental conditions, such as soil and water as well as animal-, human-, and plant-host-associated environments. The utilization of various carbon sources and energy metabolism (respiration or fermentation) might contribute to the environmental adaptation of P. aeruginosa [3]. Whereas the dehydrogenases and reductases involved in respiratory metabolism have been well described and annotated in the genome of P. aeruginosa PAO1 [4, 5], the composition of its quinone pool has not yet been fully established. Recent data indicated that the metabolic use of various quinone species according to environmental dioxygen availability is more complex than initially thought. The flavin-dependent monooxygenases UbiI, UbiF, and UbiH, which catalyze the O2-dependent hydroxylation steps, are not involved in the anaerobic pathway, and the accessory UbiK and UbiJ proteins are not implicated in the assembly and/or stability of the aerobic Ubi complex [11]. The anaerobic pathway requires UbiT, UbiU, and UbiV proteins. As explained above, the metabolic relevance of the O2-independent UQ pathway is not yet clearly understood
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