Pseudomonas stutzeri ZWLR2-1 mineralizes 2-chloronitrobenzene (2CNB), a synthetic chlorinated nitroaromatic, through a pathway assembled by patchwork recruitment: a Nag-like nitroarene dioxygenase module (cnbAaAbAcAd), whose oxygenase subunit genes cnbAcAd are carried on an IS6100-flanked transposon, oxidizes 2CNB to 3-chlorocatechol, and a tfd-like chlorocatechol degradation module (cnbCEF plus a separately recruited chloromuconate cycloisomerase gene cnbD) is predicted to convert 3-chlorocatechol to 3-oxoadipate. The initial dioxygenation and the ring-cleavage step (CnbC) are biochemically demonstrated, and the regulatory architecture of the cluster was largely settled in 2016 by the discovery of constitutive expression through one operon promoter and two internal promoters. What remains unresolved is the completion of the pathway's functional and evolutionary account: direct biochemical and in vivo validation that every step from 3-chlorocatechol to 3-oxoadipate is catalyzed by the annotated cnbCDEF gene products; reconstruction of the assembly history, including the donor lineages, order, timing, and IS6100-mediated mobilization mechanism of the recruited modules, and the residual regulatory questions about the IS6100-proximal promoter and the internal promoter P3; and whether the ecological selective pressures and fitness trade-offs fixed the pathway, which rides on an unstable megaplasmid, in the host population.

Background

2-Chloronitrobenzene (2CNB) is an anthropogenic chlorinated nitroaromatic compound used in dye, pesticide, and pharmaceutical synthesis; because it has been present in the biosphere only since the rise of industrial chemistry, bacteria able to grow on it must have assembled the degrading pathway very recently on evolutionary timescales. The first catabolic step in ZWLR2-1 is catalyzed by a nitroarene dioxygenase, a multi-component Rieske non-heme iron oxygenase system (a ferredoxin reductase, a ferredoxin, and a two-subunit oxygenase that inserts both atoms of molecular oxygen into the aromatic ring); the resulting nitrohydrodiol spontaneously eliminates nitrite, yielding 3-chlorocatechol. CnbAcAd belongs to the Nag-like subfamily of ring-hydroxylating dioxygenases, whose archetype is the naphthalene dioxygenase NagAaAbAcAd of Ralstonia sp. strain U2; Nag-like enzymes also initiate nitrobenzene, 2-nitrotoluene, and 2,4-dinitrotoluene catabolism in strains JS765, JS42, and R34/DNT. Chlorocatechols are typically metabolized by the modified ortho-cleavage pathway, homologous to the tfd genes of 2,4-dichlorophenoxyacetate degraders: a chlorocatechol 1,2-dioxygenase (TfdC-like) opens the ring to a chloro-cis,cis-muconate, a chloromuconate cycloisomerase (TfdD-like) isomerizes it with chloride elimination to a dienelactone, a dienelactone hydrolase (TfdE-like) yields maleylacetate, and a maleylacetate reductase (TfdF-like) produces 3-oxoadipate, which feeds central metabolism. In ZWLR2-1 the corresponding genes are annotated cnbC, cnbD, cnbE, and cnbF. Liu et al. (2011) cloned and sequenced a 16.2-kb region and showed that Escherichia coli expressing cnbAaAbAcAd converts 2CNB to 3-chlorocatechol with nitrite release, and that cell extracts of E. coli expressing cnbC have chlorocatechol 1,2-dioxygenase activity; the remaining lower-pathway steps were inferred only by homology (82-94% identity) to the tfdC2E2F2 cluster of Sphingomonas sp. strain TFD44, with cnbD, notably, encoding a chloromuconate cycloisomerase only distantly related (44% identity) to known CatB enzymes and sitting apart from the tfd-like block. The cluster architecture records its patchwork origin: cnbAcAd lie between two copies of the insertion sequence IS6100 (a small transposable element encoding only a transposase), forming a class I transposon identical to the catabolic transposon Tnmph of Pseudomonas sp. strain WBC-3; the tfd-like block cnbCEFAbAa sits upstream; flanking open reading frames are almost identical to plasmid Rms149 of Pseudomonas aeruginosa Ps142; and the whole cluster resides on an unstable megaplasmid. Unlike every other Nag-like dioxygenase cluster, no nagR-like transcriptional regulator gene (in U2, NagR activates the nag genes in response to salicylate) is present; Liu et al. noted a putative promoter inside the IS6100 element about 900 nucleotides upstream of cnbAc. Gao et al. (2016) subsequently showed that the cluster is expressed constitutively, not induced by 2CNB or salicylate, as a single operon driven by an upstream promoter P1 plus two internal promoters, the strongest of which (P3, directly upstream of cnbAcAd) arose by duplication of an 18-bp sequence forming its -35 region together with insertion of a 132-bp fragment identical to a segment of plasmid pAOVO02 of Acidovorax sp. strain JS42. The broader context is the patchwork-assembly model of catabolic pathway evolution (Ju and Parales 2010): new xenobiotic pathways form by recruitment of pre-existing modules through horizontal gene transfer and transposition, followed by regulatory rewiring, and analogous recently assembled pathways have been reconstructed in Diaphorobacter sp. strain JS3051 for 2,3-dichloronitrobenzene (Li et al. 2021) and in related strains. ZWLR2-1 remains one of the canonical examples of a natural 2CNB pathway, yet its lower pathway has never been biochemically verified and the donors, sequence, timing, and selective fixation of its assembly have never been established.

Problem Statement

For the 2-chloronitrobenzene catabolism pathway of Pseudomonas stutzeri ZWLR2-1, as established by Liu et al. (2011) and Gao et al. (2016), resolve the remaining mechanistic and evolutionary questions as one coherent objective: (1) determine whether each predicted step of the lower pathway from 3-chlorocatechol to 3-oxoadipate is catalyzed solely by the annotated gene products - chlorocatechol 1,2-dioxygenase CnbC forming 2-chloro-cis,cis-muconate, chloromuconate cycloisomerase CnbD, dienelactone hydrolase CnbE, and maleylacetate reductase CnbF - with the intermediates identified and the chloride and mass balance accounted for at every step; (2) reconstruct the evolutionary assembly of the cluster: identify the donor lineages of the tfd-like module cnbCEFAbAa, of the distantly related chloromuconate cycloisomerase gene cnbD, and of the IS6100-flanked cnbAcAd transposon, determine the order and approximate timing of the recruitment events and the mechanism by which IS6100-mediated transposition and other mobile elements mobilized them, and settle the residual regulatory details, namely whether the putative promoter inside the IS6100 element upstream of cnbAc is functional and whether the strong internal promoter P3 is required for growth on 2CNB in the native host; and (3) determine whether the assembled pathway's fixation in the host population was driven by selection - quantifying the ecological selective pressures and fitness trade-offs, namely the benefit of 2CNB utilization against the cost of carrying and expressing the cluster on an unstable megaplasmid - or is not distinguishable from neutral persistence. These three parts jointly answer how this recently formed xenobiotic pathway works and how it came to be. They are distinct, independently refutable sub-questions: resolving any one of them is a partial resolution of the objective, and all three together complete it.

References

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