Microbial Dehalogenase Genes Split into Mobile Respiratory Loci and a Vast Resident Reservoir

Organohalide compounds, from chlorinated solvents to pesticides, are among the most persistent pollutants in soils, sediments, and groundwater, and the microbes capable of transforming them have long fascinated environmental microbiologists. At the heart of that capability sits the reductive dehalogenase catalytic subunit, known as RdhA, an enzyme that cleaves carbon-halogen bonds and in doing so can support a specialized form of anaerobic respiration. A new genomic survey now shows that the evolutionary story of these enzymes is far more structured than previously appreciated, revealing a sharp divide between a small, highly mobile set of respiratory genes and a vast, comparatively settled reservoir of homologs whose functions remain largely unexplored.

The study, published in BMC Genomics by Boonfei Tan and Charmaine Ng of the Institutional Research Office at Manila Central University, systematically screened release 220 of the Genome Taxonomy Database, a curated collection of bacterial and archaeal genomes that spans much of known prokaryotic diversity. The search identified candidate rdhA sequences in 1,576 genomes distributed across 39 phyla. After applying motif validation filters designed to remove false positives and incomplete fragments, the researchers retained 2,392 homologs from 1,409 genomes across 36 phyla as their analytical dataset, a scale that allowed them to ask questions about mobility, taxonomy, and habitat associations that smaller surveys could not address.

Classification of these homologs by motif architecture and genomic neighbourhood revealed four distinct classes. The first and most functionally familiar group consists of canonical respiratory rdhA-rdhB loci, in which the catalytic rdhA gene is paired with rdhB, a gene encoding a membrane anchor protein, and the catalytic subunit carries a twin-arginine translocation, or TAT, signal peptide that directs export of the folded enzyme across the cytoplasmic membrane. This triad of features is the hallmark of enzymes used in organohalide respiration, a metabolism in which halogenated compounds serve as terminal electron acceptors. The second class, by far the largest, comprises solitary catabolic-like homologs lacking the rdhB partner and the canonical respiratory signature, numbering 1,845 proteins. The third class consists of tandem-array loci, with 153 proteins arranged in multiple adjacent copies, and the fourth captures other, less stereotyped architectures, accounting for 231 proteins.

The most striking finding concerns mobility. Nearly half of the canonical respiratory rdhA-rdhB loci, 48.5 percent, carried flanking mobility elements such as transposases, integrases, or other signatures of horizontal gene transfer machinery. In contrast, only 11.3 percent of the solitary catabolic-like homologs were associated with such elements. Crucially, the researchers showed that this contrast is not an artefact of taxonomy. Within Pseudomonadota, the respiratory loci carried mobility elements in 42.9 percent of cases versus 11.2 percent for catabolic-like homologs, and within Chloroflexota, a phylum famous for its dehalorespiring members, the figures were 65.5 percent versus 11.8 percent. Because the pattern repeats within individual phyla, it reflects a genuine architectural difference rather than a confounding effect of which organisms happen to carry which genes.

Horizontal transfer in the respiratory subset also reached further across the tree of life. Fifty-six percent of apparent transfers involving canonical respiratory loci crossed phylum boundaries, compared with just 13 percent for catabolic-like homologs. Once the phylum composition of the dataset was accounted for, this amounted to a 2.8-fold enrichment of cross-phylum transfer in the respiratory class. In practical terms, the genes that enable microbes to respire organohalides appear to be packaged for long-range evolutionary travel, hopping between distantly related lineages far more readily than their catabolic-like cousins, which tend to remain within the lineages where they are found.

Five independent phylogenetic tests reinforced the picture of pervasive horizontal gene transfer across the RdhA family, with the strongest discordance concentrated in the canonical respiratory subset. Duplication-transfer-loss reconciliation, a computational method that models how gene trees diverge from species trees through gene duplication, horizontal transfer, and gene loss, assigned 74 percent of inferred events as transfers. The approximately unbiased, or AU, test rejected the hypothesis that the gene tree matched the species tree, and the normalized Robinson-Foulds distance, a standard measure of topological disagreement between trees, was 0.82 on the full tree, rising to 0.90 when the analysis was restricted to the respiratory motif class. Values approaching 1.0 indicate near-complete discordance, meaning that the evolutionary history of these genes has been substantially reshuffled relative to the histories of the organisms that carry them.

The implications of this partition extend beyond evolutionary curiosity. Environmental scientists increasingly rely on metagenomics to predict whether a contaminated site can naturally attenuate chlorinated pollutants, and the presence of rdhA sequences is often taken as evidence of dechlorination potential. The new results argue that such predictions should not treat all rdhA detections as equivalent. A canonical respiratory rdhA-rdhB locus, especially one flanked by mobility elements, signals a functional capacity for organohalide respiration and a gene that may arrive, spread, or disappear dynamically within a community. A solitary catabolic-like homolog, by contrast, belongs to a broad resident reservoir whose enzymatic activities, substrates, and ecological roles are largely uncharacterized, and whose persistence within lineages suggests a more stable, possibly housekeeping-like or broadly catabolic role that has yet to be pinned down experimentally.

Tandem arrays add a third dimension to the model. The authors found that these multi-copy arrangements are associated mainly with within-lineage expansion, meaning that duplicated dehalogenase genes tend to accumulate within a single lineage rather than jumping between distant ones. Such expansions could provide raw material for functional diversification, allowing organisms to broaden the range of halogenated substrates they can process, a phenomenon previously documented in specialized organohalide-respiring bacteria that carry dozens of dehalogenase paralogs. The new survey places that observation in a genome-wide context, suggesting that tandem duplication is a distinct evolutionary channel that operates alongside, but separately from, the horizontal transfer that drives the spread of respiratory loci.

The breadth of the survey is itself notable. Detecting validated rdhA homologs across 36 phyla underscores how deeply halogen-transforming potential is woven into prokaryotic life, extending well beyond the handful of cultured dehalorespiring genera, such as Dehalococcoides and other Chloroflexota members, that have dominated laboratory studies. Most of the homologs in the resident reservoir come from organisms that have never been isolated in culture, and their catalytic diversity hints at a largely unmapped chemical space of natural and anthropogenic halogenated compounds that microbes already possess the tools to transform. As genomic databases continue to grow, the architectural classification established here offers a practical framework for interpreting new detections, distinguishing the mobile respiratory specialists that matter most for bioremediation from the quieter, resident enzymes whose functions await experimental exploration.

Subject of Research: Comparative genomics of reductive dehalogenase homologs and their horizontal transfer across prokaryotes

Article Title: Halogen-transforming potential in prokaryotes partitions into mobile respiratory loci and a broad resident reservoir

Article References: Tan, B., & Ng, C. (2026). Halogen-transforming potential in prokaryotes partitions into mobile respiratory loci and a broad resident reservoir. BMC Genomics. https://doi.org/10.1186/s12864-026-13358-7

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13358-7

Keywords: reductive dehalogenase, organohalide respiration, horizontal gene transfer, comparative genomics, GTDB, phylogenomics, mobile genetic elements, microbial ecology, bioremediation, RdhA, RdhB, dechlorination