Different Functions of Phylogenetically Distinct Bacterial Complex I Isozymes

Abstract

NADH:quinone oxidoreductase (complex I) is a bioenergetic enzyme that transfers electrons from NADH to quinone, conserving the energy of this reaction by contributing to the proton motive force. While the importance of NADH oxidation to mitochondrial aerobic respiration is well documented, the contribution of complex I to bacterial electron transport chains has been tested in only a few species. Here, we analyze the function of two phylogenetically distinct complex I isozymes in Rhodobacter sphaeroides, an alphaproteobacterium that contains well-characterized electron transport chains. We found that R. sphaeroides complex I activity is important for aerobic respiration and required for anaerobic dimethyl sulfoxide (DMSO) respiration (in the absence of light), photoautotrophic growth, and photoheterotrophic growth (in the absence of an external electron acceptor). Our data also provide insight into the functions of the phylogenetically distinct R. sphaeroides complex I enzymes (complex IA and complex IE) in maintaining a cellular redox state during photoheterotrophic growth. We propose that the function of each isozyme during photoheterotrophic growth is either NADH synthesis (complex IA) or NADH oxidation (complex IE). The canonical alphaproteobacterial complex I isozyme (complex IA) was also shown to be important for routing electrons to nitrogenase-mediated H2 production, while the horizontally acquired enzyme (complex IE) was dispensable in this process. Unlike the singular role of complex I in mitochondria, we predict that the phylogenetically distinct complex I enzymes found across bacterial species have evolved to enhance the functions of their respective electron transport chains.IMPORTANCE Cells use a proton motive force (PMF), NADH, and ATP to support numerous processes. In mitochondria, complex I uses NADH oxidation to generate a PMF, which can drive ATP synthesis. This study analyzed the function of complex I in bacteria, which contain more-diverse and more-flexible electron transport chains than mitochondria. We tested complex I function in Rhodobacter sphaeroides, a bacterium predicted to encode two phylogenetically distinct complex I isozymes. R. sphaeroides cells lacking both isozymes had growth defects during all tested modes of growth, illustrating the important function of this enzyme under diverse conditions. We conclude that the two isozymes are not functionally redundant and predict that phylogenetically distinct complex I enzymes have evolved to support the diverse lifestyles of bacteria.

Tracking Electron Uptake from a Cathode into Shewanella Cells: Implications for Energy Acquisition from Solid-Substrate Electron Donors

“…6C ). Complex I is a reversible proton-pumping NADH/ubiquinone-oxidoreductase ( 41 , 42 ) that has been implicated in reverse electron flow in other organisms ( 34 , 35 , 43 , 44 ). In the Δ nuo-lux strain, steady light production was observed for a >5-h period until ultimately declining, consistent with the ultimate decline in cell health resulting from CCCP addition.…”

Comparative genomics sheds light on niche differentiation and the evolutionary history of comammox Nitrospira

“…This type of complex I has also been found in other distantly related organisms, including Nitrosospira multiformis and some Bacteroidetes spp., suggesting acquisition by horizontal gene transfer [47]. Although the exact function of this additional complex I is not known, differential expression of distinct complex I isozymes has been observed in other microorganisms, pointing towards physiological versatility [48,49]. Also, differences with respect to complex II, the succinate:quinone oxidoreductase (SQR), were noted across genomes.…”

NADH Dehydrogenases in Pseudomonas aeruginosa Growth and Virulence

“…The P. aeruginosa genome encodes at least three bioinformatically predicted NADH dehydrogenases (NADH:quinone oxidoreductases). NDH-1, encoded by the nuoA-N operon (PA2637-2649), is homologous to the mitochondrial complex I, and has a fused nuoCD subunit (Spero et al, 2016). NDH-1 enzymes both translocate protons and oxidize NADH to NAD + (Williams et al, 2007).…”