Characterization of two regulatory genes of the mercury resistance determinants from TnMERI1 by luciferase-based examination

Adsorption of Hg2+/Cr6+ by metal-binding proteins heterologously expressed in Escherichia coli

BACKGROUND

Removal of heavy metals from water and soil is a pressing challenge in environmental engineering, and biosorption by microorganisms is considered as one of the most cost-effective methods. In this study, the metal-binding proteins MerR and ChrB derived from Cupriavidus metallidurans were separately expressed in Escherichia coli BL21 to construct adsorption strains. To improve the adsorption performance, surface display and codon optimization were carried out.

RESULTS

In this study, we constructed 24 adsorption engineering strains for Hg2+ and Cr6+, utilizing different strategies. Among these engineering strains, the M'-002 and B-008 had the strongest heavy metal ion absorption ability. The M'-002 used the flexible linker and INPN to display the merRopt at the surface of the E. coli BL21, whose maximal adsorption capacity reached 658.40 μmol/g cell dry weight under concentrations of 300 μM Hg2+. And the B-008 overexpressed the chrB in the intracellular, its maximal capacity was 46.84 μmol/g cell dry weight under concentrations 500 μM Cr6+. While in the case of mixed ions solution (including Pb2+, Cd2+, Cr6+ and Hg2+), the total amount of ions adsorbed by M'-002 and B-008 showed an increase of up to 1.14- and 4.09-folds, compared to the capacities in the single ion solution.

CONCLUSION

The construction and optimization of heavy metal adsorption strains were carried out in this work. A comparison of the adsorption behavior between single bacteria and mixed bacteria systems was investigated in both a single ion and a mixed ion environment. The Hg2+ absorption capacity is reached the highest reported to date with the engineered strain M'-002, which displayed the merRopt at the surface of chassis cell, indicating the strain's potential for its application in practical environments.

An improved direct metamobilome approach increases the detection of larger-sized circular elements across kingdoms

Mobile genetic elements (MGEs) are instrumental in natural prokaryotic genome editing, permitting genome plasticity and allowing microbes to accumulate genetic diversity. MGEs serve as a vast communal gene pool and include DNA elements such as plasmids and bacteriophages (phages) among others. These mobile DNA elements represent a human health risk as they can introduce new traits, such as antibiotic resistance or virulence, to a bacterial strain. Sequencing libraries targeting environmental circular MGEs, referred to as metamobilomes, may broaden our current understanding of the mechanisms behind the mobility, prevalence and content of these elements. However, metamobilomics is affected by a severe bias towards small circular elements, introduced by multiple displacement amplification (MDA). MDA is typically used to overcome limiting DNA quantities after the removal of non-circular DNA during library preparations. By examining the relationship between sequencing coverage and the size of circular MGEs in paired metamobilome datasets with and without MDA, we show that larger circular elements are lost when using MDA. This study is the first to systematically demonstrate that MDA is detrimental to detecting larger-sized plasmids if small plasmids are present. It is also the first to show that MDA can be omitted when using enzyme-based DNA fragmentation and PCR in library preparation kits such as Nextera XT® from Illumina.

Metagenomic Evidence for a Methylocystis Species Capable of Bioremediation of Diverse Heavy Metals

Heavy metal pollution has become an increasingly serious problem worldwide. Co-contamination with toxic mercury (Hg) and arsenic (As) presents a particularly difficult bioremediation trouble. By oxidizing the greenhouse gas methane, methanotrophs have been demonstrated to have high denitrification activity in eutrophic waters, indicating their possible potential for use in bioremediation of Hg(II) and As(V) in polluted water. Using metagenomics, a novel Methylocystis species (HL18), which was one of the most prevalent bacteria (9.9% of the relative abundance) in a CH₄-based bio-reactor, is described here. The metagenomic-assembled genome (MAG) HL18 had gene products whose average amino acid identity against other known Methylocystis species varied from 69 to 85%, higher than the genus threshold but lower than the species boundary. Genomic analysis indicated that HL18 possessed all the genes necessary for the reduction of Hg(II) and As(V). Phylogenetic investigation of mercuric reductase (MerA) found that the HL18 protein was most closely affiliated with proteins from two Hg(II)-reducing bacteria, Bradyrhizobium sp. strain CCH5-F6 and Paracoccus halophilus. The genomic organization and phylogeny of the genes in the As(V)-reducing operon (arsRCCB) had significant identity with those from a As(V)-reducing bacterium belonging to the Rhodopseudomonas genus, indicating their reduction capability of As(V). Further analysis found that at least eight genera of methanotrophs possess both Hg(II) and As(V) reductases, illustrating the generally overlooked metabolic potential of methanotrophs. These results suggest that methanotrophs have greater bioremediation potential in heavy metal contaminated water than has been appreciated and could play an important role in the mitigation of heavy metal toxicity of contaminated wastewater.

Structural basis of the mercury(II)-mediated conformational switching of the dual-function transcriptional regulator MerR

The mer operon confers bacterial resistance to inorganic mercury (Hg(2+)) and organomercurials by encoding proteins involved in sensing, transport and detoxification of these cytotoxic agents. Expression of the mer operon is under tight control by the dual-function transcriptional regulator MerR. The metal-free, apo MerR binds to the mer operator/promoter region as a repressor to block transcription initiation, but is converted into an activator upon Hg(2+)-binding. To understand how MerR interacts with Hg(2+) and how Hg(2+)-binding modulates MerR function, we report here the crystal structures of apo and Hg(2+)-bound MerR from Bacillus megaterium, corresponding respectively to the repressor and activator conformation of MerR. To our knowledge, the apo-MerR structure represents the first visualization of a MerR family member in its intact and inducer-free form. And the Hg(2+)-MerR structure offers the first view of a triligated Hg(2+)-thiolate center in a metalloprotein, confirming that MerR binds Hg(2+) via trigonal planar coordination geometry. Structural comparison revealed the conformational transition of MerR is coupled to the assembly/disassembly of a buried Hg(2+) binding site, thereby providing a structural basis for the Hg(2+)-mediated functional switching of MerR. The pronounced Hg(2+)-induced repositioning of the MerR DNA-binding domains suggests a plausible mechanism for the transcriptional regulation of the mer operon.

Expressing a bacterial mercuric ion binding protein in plant for phytoremediation of heavy metals

A specific mercuric ion binding protein (MerP) originating from transposon TnMERI1 of Bacillus megaterium strain MB1 isolated from Minamata Bay displayed good adsorption capability for a variety of heavy metals. In this study, the Gram-positive MerP protein was expressed in transgenic Arabidopsis to create a model system for phytoremediation of heavy metals. Under control of an actin promoter, the transgenic Arabidpsis showed higher tolerance and accumulation capacity for mercury, cadium and lead when compared with the control plant. Results from confocal microscopy analysis also indicate that MerP was localized at the cell membrane and vesicles of plant cells. The developed transgenic plants possessing excellent metal-accumulative ability could have potential applications in decontamination of heavy metals.