When bacteria acquire the capacity to aerobically degrade a xenobiotic they have never metabolized before, the early evolutionary steps typically run through faulty oxygenation reactions that release reactive oxygen species (ROS). Akkaya et al. (mBio 2018) showed that Pseudomonas putida, whose central metabolism maintains a highly reductive NAD(P)H regime, buffers this endogenous ROS so that catabolism of the nitroaromatic 2,4-dinitrotoluene does not trigger mutagenesis, whereas artificially lowering the NAD(P)H pool with a water-forming NADH oxidase converts the same catabolism into a conspicuous mutagenic and genomic-diversification regime. The paper generalizes this into a principle — the metabolic redox regime tunes evolvability — and raises four follow-up questions that remain open: (i) which bacterial taxa beyond Pseudomonas and Burkholderia buffer ROS through high NAD(P)H fluxes and which enter mutagenic regimes; (ii) which molecular mechanisms account for the protection afforded by elevated NAD(P)H; (iii) whether 'innovation specialists' in multispecies consortia generate adaptive diversity that is horizontally transferred to less mutable members; and (iv) whether deliberate perturbation of NAD(P)+/NAD(P)H balance can modulate the rate of emergence of antibiotic resistance and whether ROS-mediated diversification can be harnessed as a directed-evolution tool.

Background

A xenobiotic is a synthetic compound foreign to natural biochemistry, such as the munitions-derived nitroaromatic 2,4-dinitrotoluene (2,4-DNT). Some environmental bacteria evolve new aerobic biodegradation pathways for such compounds by recruiting preexisting enzymes, most often Rieske nonheme iron oxygenases — multi-component enzyme systems that activate molecular oxygen to hydroxylate an aromatic ring through a [2Fe-2S] cluster and a mononuclear iron center. The critical bottleneck is the first oxygenation step: when the substrate fits the enzyme active site poorly, oxygen activation becomes uncoupled from substrate turnover and electrons leak to oxygen, releasing reactive oxygen species (ROS) such as hydrogen peroxide, superoxide, and hydroxyl radical. ROS damage DNA, proteins, and membranes; the oxidation product 8-oxo-guanine (8-oxoG) mispairs with adenine and produces characteristic G:C-to-T:A (reported as C-to-A) transversions. In Burkholderia sp. R34, the original host of the 2,4-DNT pathway (whose evolutionary origin was traced by Johnson, Jain and Spain in 2002), Pérez-Pantoja, Nikel, Chavarría and de Lorenzo showed in 2013 that this endogenous ROS kills most of the population but drives genetic diversification of the survivors — endogenous stress as an engine of pathway evolution. Akkaya, Pérez-Pantoja, Calles, Nikel and de Lorenzo (mBio 2018) then implanted the complete dnt pathway from Burkholderia sp. R34 into Pseudomonas putida EM173 (a genome-reduced derivative of the soil bacterium and metabolic-engineering chassis P. putida KT2440). Catabolism of 2,4-DNT in this engineered strain raised intracellular ROS roughly 10-fold, yet the SOS response (the recA/lexA-controlled emergency program for DNA-damage tolerance in bacteria) was not significantly activated and the genomic mutation rate — quantified by Luria-Delbrück fluctuation analysis, in which the rate of spontaneous mutants is estimated from the distribution of rifampin-resistant colonies across parallel cultures (resistance being conferred by point mutations in the rpoB gene encoding the RNA polymerase beta subunit) — increased by a statistically insignificant 28%. The authors attributed this buffering to the distinctive redox regime of P. putida: its central carbon metabolism operates the EDEMP cycle, a cyclic arrangement of Entner-Doudoroff, Embden-Meyerhof-Parnas, and pentose-phosphate pathway reactions that generates surplus NADPH, the reduced pyridine nucleotide that fuels ROS-detoxifying systems. Crucially, when they lowered the NAD(P)H pool by expressing a water-forming, NADH-specific oxidase (Nox), the same 2,4-DNT catabolism produced a 6-fold increase in mutation rate and an elevated frequency of 8-oxoG-signature C-to-A transversions in rpoB, i.e., a mutagenic, diversity-generating regime like the one seen in Burkholderia. The paper frames this as evolutionary hormesis — a productive optimum in the novelty supply rate (the rate at which a population generates heritable novelty) between too little diversification and lethal damage — and concludes that the background metabolic network, through its redox regime, determines whether an evolving catabolic pathway is silently hosted or drives the host into genetic diversification. The mechanistic discussion leaves open which components account for the protection: direct detoxification of ROS by NAD(P)H-fueled systems, the DNA base-excision-repair and nucleotide-sanitization enzymes that remove oxidized bases, or stress-response-mediated relaxation of replication fidelity and mismatch repair (the proofreading system that corrects replication errors, and which general stress signaling can transiently inhibit). In the source paper these follow-up items are raised separately rather than as parts of one required deliverable: the Discussion notes that data supporting the principle 'have been generated in only two environmental bacteria (Pseudomonas putida and Burkholderia spp.)' although they are 'likely ... specific cases of a general evolutionary principle'; the Results ask 'Yet, what could such key factors be?' about the protection; and the closing Perspectives lists illustrative corollaries 'worth follow-up' — that ROS may accelerate the emergence of new antibiotic resistances, opening the possibility of targeting the redox biochemistry of virulent strains with drugs that perturb the NAD(P)+/NAD(P)H balance; that there could be 'innovation specialists' nurturing genetic diversification and passing the results to less mutable members of a multispecies bacterial consortium through horizontal gene transfer; and that ROS-mediated genetic innovation could be repurposed for generating new pathways through directed evolution or for the optimization of biocatalytic routes. These passages define the open problem below.

Problem Statement

The problem, as posed by Akkaya et al. 2018 (Discussion and closing Perspectives), is to establish the principle that the metabolic redox regime of a bacterium controls whether reactive oxygen species released during faulty catabolism of a novel xenobiotic are coupled to DNA mutagenesis — and to work out that principle's generality, mechanism, ecological consequences, and exploitable applications. It comprises four linked questions bearing on this objective, each of which remains open: (1) Generality — determine, across diverse bacterial taxa beyond the two environmental bacteria in which it has been examined (Pseudomonas putida and Burkholderia spp., plus the supporting Escherichia coli experiment of Akkaya et al. 2019), which phylogenetic groups buffer endogenous ROS through high NAD(P)H fluxes and which enter a mutagenic diversification regime when hosting an evolving pathway for a novel substrate; (2) Mechanism — identify the molecular components that account for the protection afforded by elevated NAD(P)H, i.e., the respective roles of NAD(P)H-fueled detoxification systems, nucleotide-sanitization and DNA-repair enzymes, and stress-response-mediated modulations of replication fidelity and mismatch repair, and how the protection depends on NAD(P)H availability; (3) Community consequences — test whether members of multispecies consortia with innately more mutagenic (weak-redox) metabolisms act as 'innovation specialists' that generate adaptive genetic diversity which is then horizontally transferred to less mutable community members; (4) Translational corollaries — test whether deliberate perturbation of the NAD(P)+/NAD(P)H balance in clinically relevant strains modulates the rate at which antibiotic resistance emerges, and whether ROS-mediated genetic diversification can be harnessed as a directed-evolution tool to generate or optimize whole-cell biocatalysts. Each of these four questions is independently refutable and carries its own completion boundary: resolving any one of them is a self-contained contribution to the objective and leaves the other three open, and the objective as a whole is established only when all four are resolved.

References

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