Researchers have successfully localized the binding regions for the ParB protein within the pHG1 megaplasmid of the bacterium Cupriavidus necator H16, a critical step toward developing antibiotic-free plasmid vectors for industrial bioproduction. Published in the Annals of Microbiology, this study provides the first in vitro empirical evidence identifying two distinct ParB-binding regions within the origin-proximal segment of the megaplasmid. The work addresses a significant gap in the genetic engineering of C. necator H16, a premier lithoautotrophic strain widely utilized for the production of polyhydroxyalkanoate (PHA) bioplastics. By mapping these regions, the study establishes a biochemical foundation for engineering stable, selection-free expression vectors that can maintain plasmids without the continuous use of expensive antibiotic markers.
The industrial deployment of recombinant C. necator strains is currently hindered by the reliance on conventional plasmid vectors that require continuous antibiotic selection to prevent segregational plasmid loss during large-scale fermentation. This dependency renders industrial bioproduction economically impractical due to the metabolic and financial burdens associated with antibiotic supplementation. To circumvent these limitations, researchers have turned to the native active partitioning (ParABS) system found in the C. necator H16 megaplasmid pHG1. This system ensures faithful plasmid inheritance by actively coordinating the migration of replicated plasmid copies to opposite cell poles prior to cell division, offering a promising selection-free alternative for stable plasmid maintenance.
While the trans-acting motor protein ParA and the centromere-binding protein ParB of pHG1 were previously annotated in genomic studies, the essential cis-acting centromere sequence, known as parS, remained unmapped. The lack of a defined parS locus prevented the isolation of a complete, functional partitioning cassette required for vector development. The objective of this study was to bioinformatically classify the components of the pHG1 ParABS system and to experimentally localize the ParB-binding regions in vitro. By overexpressing an N-terminal hexahistidine-tagged ParB and employing affinity chromatography against distinct DNA segments of the pHG1 origin-proximal region, the researchers aimed to identify the functional parS loci necessary for constructing robust vectors.
Bioinformatic analysis of the origin-proximal region of the pHG1 megaplasmid indicated that the parA and parB genes are adjacently located, characterized by a putative single promoter upstream of the operon. Motif search revealed that ParA contains conserved AbiEi_4 transcriptional regulator, P-loop, and ATPase motifs, while ParB harbors the conserved ParBc motif and a central Helix-Turn-Helix DNA-binding motif. These specific domain architectures identify the pHG1 parAB operon as a prototypical Type Ia active partitioning system. However, protein alignments of pHG1 ParB against six known ParB homologs with identified cognate parS sequences demonstrated poor conservation, yielding only 26% to 45% sequence identity. Consequently, the PAR region failed to align with any of the six evaluated parS reference sequences, suggesting that the pHG1 parS may belong to a previously uncharacterized or highly divergent subset.
To functionally localize the centromeric loci, the researchers developed an in vitro affinity chromatography assay utilizing an immobilized, recombinant N-terminal hexahistidine-tagged ParB, referred to as H6ParB. The H6ParB protein was successfully cloned and overexpressed in Escherichia coli BL21(DE3) under cold-shock conditions, yielding a distinct band corresponding to the expected molecular weight of approximately 39.2 kDa. The protein partitioned into the soluble fraction and exhibited strong and specific affinity for the Ni-NTA resin during immobilized metal affinity chromatography purification.
The affinity chromatography assay tested the binding capability of immobilized H6ParB against specific DNA fragments, including the approximately 2.7 kb PAR fragment and the upstream approximately 4.6 kb ORI fragment. Agarose gel electrophoresis of the eluate fractions demonstrated the strong retention of the approximately 2.7 kb PAR fragment, indicating that a putative primary parS locus is physically located within the PAR region immediately downstream of the parAB operon. This regional localization aligns with the bioinformatic prediction of the pHG1 partitioning module as a Type Ia ParABS system, which typically features a downstream centromere orientation. The retention of this fragment suggests that it represents a complete and modular stabilizing cassette, inherently encompassing the native promoter, parA, and parB genes.
Interestingly, the assay also revealed a distinct, albeit much fainter, band corresponding to the approximately 4.6 kb ORI fragment in its respective eluate. This observation implies that H6ParB forms a specific, yet weaker, nucleoprotein complex within the ORI region, suggesting the presence of a putative secondary, low-affinity parS site. The presence of multiple parS loci with varying binding affinities is a documented phenomenon in several well-characterized partitioning systems, such as the P1 plasmid and the Bacillus subtilis chromosome. These secondary loci frequently function in a cooperative manner to assist in the non-specific binding, nucleation, spreading, and oligomerization of ParB over the plasmid DNA, which is essential for the maturation of the segrosome partition complex.
To confirm that the retention was strictly due to specific sequence recognition, linearized non-specific control plasmids, pHSG398 and pBBR1MCS, were evaluated under identical assay conditions. Both negative controls failed to yield specific bands in the final eluate, supporting the sequence-specificity of the H6ParB binding assay. Trace amounts of high-molecular-weight background were noted across assays, representing minor non-specific affinities with sheared host chromosomal DNA, but this did not interfere with the specific retention profiles of the target fragments. The lack of sequence homology to known parS consensus sequences highlights the evolutionary divergence necessary to maintain segregational specificity and prevent plasmid incompatibility within the multipartite genome of C. necator H16, which harbors two chromosomes alongside the pHG1 megaplasmid.
Despite the successful regional mapping of these interactions, the study acknowledges a methodological limitation in terms of molecular resolution. The reliance on distinct macroscopic DNA fragments to assess modular cassette functionality means that the in vitro pull-down assay lacks the base-pair resolution necessary to definitively attribute binding to specific palindromic motifs. Consequently, these qualitative binding observations must eventually be supplemented by quantitative kinetic evaluations, such as Kd determinations, and high-resolution mapping techniques like chromatin immunoprecipitation sequencing or electrophoretic mobility shift assays. The proposition of a novel parS consensus sequence remains strictly putative until confirmed by these high-resolution methods, which will be necessary to precisely define the functional palindromic motif.
Biotechnologically, localizing the primary parS within the continuous approximately 2.7 kb PAR fragment has profound implications for the industrial-scale bioproduction of PHA. Integrating this native ParABS system into expression vectors offers a highly promising and antibiotic-free strategy to ensure plasmid maintenance during fermentation. To fully understand this application, future in vivo segregational stability assays must be executed in a pHG1-cured C. necator host to prevent native plasmid incompatibility and empirically validate the cassette’s stabilizing efficacy. Ultimately, the identification of the continuous PAR region as a self-contained partitioning module lays the essential biochemical groundwork for engineering stable, selection-free vectors, representing a vital advancement toward the sustainable, industrial-scale bioproduction of PHA bioplastics.
Subject of Research: In vitro localization of ParB-binding regions in the pHG1 megaplasmid partitioning system of Cupriavidus necator H16
Article Title: In vitro localization of ParB-binding regions within the megaplasmid pHG1 partitioning system of Cupriavidus necator H16
Article References: Chee, P. K., Suzazsuzuludin, A., Samian, M. R., Salim, H., & Zarkasi, K. Z. (2026). In vitro localization of ParB-binding regions within the megaplasmid pHG1 partitioning system of Cupriavidus necator H16. Annals of Microbiology, 76(1), Article 39. https://doi.org/10.1186/s13213-026-01865-7
Image Credits: AI Generated
DOI: 10.1186/s13213-026-01865-7
Keywords: Cupriavidus necator, pHG1 megaplasmid, ParABS system, ParB binding, plasmid partitioning, polyhydroxyalkanoate, affinity chromatography, bioproduction, selection-free vectors, Type Ia partitioning, vitro, localization
News Source: Drew Townsend. (November 8, 2026). Researchers Map ParB Binding Regions in pHG1 Megaplasmid. Scienmag.
