Construction of a broad-host-range Anderson promoter series and particulate methane monooxygenase promoter variants expand the methanotroph genetic toolbox

Methanotrophic bacteria are currently used industrially for the bioconversion of methane-rich natural gas and anaerobic digestion-derived biogas to valuable products. These bacteria may also serve to mitigate the negative effects of climate change by capturing atmospheric greenhouse gases. Several genetic tools have previously been developed for genetic and metabolic engineering of methanotrophs. However, the available tools for use in methanotrophs are significantly underdeveloped compared to many other industrially relevant bacteria, which hinders genetic and metabolic engineering of these biocatalysts. As such, expansion of the methanotroph genetic toolbox is needed to further our understanding of methanotrophy and develop biotechnologies that leverage these unique microbes for mitigation and conversion of methane to valuable products. Here, we determined the copy number of three broad-host-range plasmids in Methylococcus capsulatus Bath and Methylosinus trichosporium OB3b, representing phylogenetically diverse Gammaproteobacterial and Alphaproteobacterial methanotrophs, respectively. Further, we show that the commonly used synthetic Anderson series promoters are functional and exhibit similar relative activity in M. capsulatus and M. trichosporium OB3b, but the synthetic series had limited range. Thus, we mutagenized the native M. capsulatus particulate methane monooxygenase promoter and identified variants with activity that expand the activity range of synthetic, constitutive promoters functional not only in M. capsulatus, but also in Escherichia coli. Collectively, the tools developed here advance the methanotroph genetic engineering toolbox and represent additional synthetic genetic parts that may have broad applicability in Pseudomonadota bacteria.

Unique high Arctic methane metabolizing community revealed through in situ 13CH4-DNA-SIP enrichment in concert with genome binning

Greenhouse gas (GHG) emissions from Arctic permafrost soils create a positive feedback loop of climate warming and further GHG emissions. Active methane uptake in these soils can reduce the impact of GHG on future Arctic warming potential. Aerobic methane oxidizers are thought to be responsible for this apparent methane sink, though Arctic representatives of these organisms have resisted culturing efforts. Here, we first used in situ gas flux measurements and qPCR to identify relative methane sink hotspots at a high Arctic cytosol site, we then labeled the active microbiome in situ using DNA Stable Isotope Probing (SIP) with heavy 13CH4 (at 100 ppm and 1000 ppm). This was followed by amplicon and metagenome sequencing to identify active organisms involved in CH4 metabolism in these high Arctic cryosols. Sequencing of 13C-labeled pmoA genes demonstrated that type II methanotrophs (Methylocapsa) were overall the dominant active methane oxidizers in these mineral cryosols, while type I methanotrophs (Methylomarinovum) were only detected in the 100 ppm SIP treatment. From the SIP-13C-labeled DNA, we retrieved nine high to intermediate quality metagenome-assembled genomes (MAGs) belonging to the Proteobacteria, Gemmatimonadetes, and Chloroflexi, with three of these MAGs containing genes associated with methanotrophy. A novel Chloroflexi MAG contained a mmoX gene along with other methane oxidation pathway genes, identifying it as a potential uncultured methane oxidizer. This MAG also contained genes for copper import, synthesis of biopolymers, mercury detoxification, and ammonia uptake, indicating that this bacterium is strongly adapted to conditions in active layer permafrost and providing new insights into methane biogeochemical cycling. In addition, Betaproteobacterial MAGs were also identified as potential cross-feeders with methanotrophs in these Arctic cryosols. Overall, in situ SIP labeling combined with metagenomics and genome binning demonstrated to be a useful tool for discovering and characterizing novel organisms related to specific microbial functions or biogeochemical cycles of interest. Our findings reveal a unique and active Arctic cryosol microbial community potentially involved in CH4 cycling.

Optimized Tools and Methods for Methanotroph Genome Editing

Microbes with the capacity to use methane (CH₄) as a carbon source (methanotrophs) have significant potential for the bioconversion of CH₄-containing natural gas and anaerobic digestion-derived biogas to high value products. These organisms also play a vital role in the biogeochemical cycling of atmospheric CH₄ by serving as the only known biological sink of this gas in terrestrial and aquatic ecosystems. Much is known regarding the enzymes and central metabolic pathways mediating CH₄ utilization in these bacteria. However, large fundamental knowledge gaps exist regarding methanotroph physiology and responses to environmental stimuli, primarily due to a lack of efficient molecular tools to probe gene-function relationships. In this chapter, we describe several recently developed genetic tools and optimized genome editing methods that can be used for methanotroph metabolic engineering and to probe metabolic and physiological governing mechanisms in these unique bacteria.

The effect of lanthanum on growth and gene expression in a facultative methanotroph

The biological importance of lanthanides has only recently been identified, initially as the active site metal of the alternative methanol dehydrogenase (MDH) Xox‐MDH. So far, the effect of lanthanide (Ln) has only been studied in relatively few organisms. This work investigated the effects of Ln on gene transcription and protein expression in the facultative methanotroph Methylocella silvestris BL2, a widely distributed methane‐oxidizing bacterium with the unique ability to grow not just on methane but also on other typical components of natural gas, ethane and propane. Expression of calcium‐ or Ln‐dependent MDH was controlled by Ln (the lanthanide switch) during growth on one‐, two‐ or three‐carbon substrates, and Ln imparted a considerable advantage during growth on propane, a novel result extending the importance of Ln to consumers of this component of natural gas. Two Xox‐MDHs were expressed and regulated by Ln in M. silvestris, but interestingly Ln repressed rather than induced expression of the second Xox‐MDH. Despite the metabolic versatility of M. silvestris, no other alcohol dehydrogenases were expressed, and in double‐mutant strains lacking genes encoding both Ca‐ and Ln‐dependent MDHs (mxaF and xoxF5 or xoxF1), growth on methanol and ethanol appeared to be enabled by expression of the soluble methane monooxygenase.

Development of a CRISPR/Cas9 System for Methylococcus capsulatus In Vivo Gene Editing

In this study, we targeted the development and evaluation of broad-host-range CRISPR/Cas9 gene-editing tools in order to enhance the genetic-engineering capabilities of an industrially relevant methanotrophic biocatalyst. The CRISPR/Cas9 system developed in this study expands the genetic tools available to define molecular mechanisms in methanotrophic bacteria and has the potential to foster advances in the generation of novel biocatalysts to produce biofuels, platform chemicals, and high-value products from natural gas- and biogas-derived methane. Further, due to the broad-host-range applicability, these genetic tools may also enable innovative approaches to overcome the barriers associated with genetically engineering diverse, industrially promising nonmodel microorganisms.

Methanotrophic bacteria play a crucial role in the Earth’s biogeochemical cycle and have the potential to be employed in industrial biomanufacturing processes due to their capacity to use natural gas- and biogas-derived methane as a sole carbon and energy source. Advanced gene-editing systems have the potential to enable rapid, high-throughput methanotrophic genetics and biocatalyst development. To this end, we employed a series of broad-host-range expression plasmids to construct a conjugatable clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene-editing system in Methylococcus capsulatus (Bath). Heterologous coexpression of the Streptococcus pyogenes Cas9 endonuclease and a synthetic single guide RNA (gRNA) showed efficient Cas9 DNA targeting and double-stranded DNA (dsDNA) cleavage that resulted in cell death. We demonstrated effective in vivo editing of plasmid DNA using both Cas9 and Cas9^D10A nickase to convert green fluorescent protein (GFP)- to blue fluorescent protein (BFP)-expressing cells with 71% efficiency. Further, we successfully introduced a premature stop codon into the soluble methane monooxygenase (sMMO) hydroxylase component-encoding mmoX gene with the Cas9^D10A nickase, disrupting sMMO function. These data provide proof of concept for CRISPR/Cas9-mediated gene editing in M. capsulatus. Given the broad-host-range replicons and conjugation capability of these CRISPR/Cas9 tools, they have potential utility in other methanotrophs and a wide array of Gram-negative microorganisms.

IMPORTANCE In this study, we targeted the development and evaluation of broad-host-range CRISPR/Cas9 gene-editing tools in order to enhance the genetic-engineering capabilities of an industrially relevant methanotrophic biocatalyst. The CRISPR/Cas9 system developed in this study expands the genetic tools available to define molecular mechanisms in methanotrophic bacteria and has the potential to foster advances in the generation of novel biocatalysts to produce biofuels, platform chemicals, and high-value products from natural gas- and biogas-derived methane. Further, due to the broad-host-range applicability, these genetic tools may also enable innovative approaches to overcome the barriers associated with genetically engineering diverse, industrially promising nonmodel microorganisms.

Chalkophores

Copper-binding metallophores, or chalkophores, play a role in microbial copper homeostasis that is analogous to that of siderophores in iron homeostasis. The best-studied chalkophores are members of the methanobactin (Mbn) family-ribosomally produced, posttranslationally modified natural products first identified as copper chelators responsible for copper uptake in methane-oxidizing bacteria. To date, Mbns have been characterized exclusively in those species, but there is genomic evidence for their production in a much wider range of bacteria. This review addresses the current state of knowledge regarding the function, biosynthesis, transport, and regulation of Mbns. While the roles of several proteins in these processes are supported by substantial genetic and biochemical evidence, key aspects of Mbn manufacture, handling, and regulation remain unclear. In addition, other natural products that have been proposed to mediate copper uptake as well as metallophores that have biologically relevant roles involving copper binding, but not copper uptake, are discussed.

In situ real-time evaluation of radiation-responsive promoters in the extremely radioresistant microbe Deinococcus radiodurans

A third generation promoter probe shuttle vector pKG was constructed, using the green fluorescent protein as a reporter, for in situ evaluation of Deinococcal promoter activity in Escherichia coli or Deinococcus radiodurans. The construct yielded zero background fluorescence in both the organisms, in the absence of promoter sequences. Fifteen Deinococcal promoters, either harbouring Radiation and Desiccation Response Motif (RDRM) or not, were cloned in vector pKG. Only the RDRM-promoter constructs displayed (i) gamma radiation inducible GFP expression in D. radiodurans, following gamma irradiation, (ii) DdrO-mediated repression of GFP expression in heterologous E. coli, or (iii) abolition in GFP induction following gamma irradiation, in pprI mutant of D. radiodurans. Utility of pKG vector for real-time in situ assessment of Deinococcal promoter function was, thus, successfully demonstrated.

Copper-responsive gene expression in the methanotroph Methylosinus trichosporium OB3b

Methanotrophic bacteria convert methane to methanol using methane monooxygenase (MMO) enzymes. In many strains, either an iron-containing soluble (sMMO) or a copper-containing particulate (pMMO) enzyme can be produced depending on copper availability; the mechanism of this copper switch has not been elucidated. A key player in methanotroph copper homeostasis is methanobactin (Mbn), a ribosomally produced, post-translationally modified natural product with a high affinity for copper. The Mbn precursor peptide is encoded within an operon that contains a range of putative transporters, regulators, and biosynthetic proteins, but the involvement of these genes in Mbn-related processes remains unclear. Extensive time-dependent qRT-PCR studies of Methylosinus trichosporium OB3b and the constitutive sMMO-producing mutant M. trichosporium OB3b PP358 show that the Mbn operon is indeed copper-regulated, providing experimental support for its bioinformatics-based identification. Moreover, the Mbn operon is co-regulated with the sMMO operon and reciprocally regulated with the pMMO operon. Within the Mbn and sMMO operons, a subset of regulatory genes exhibits a distinct and shared pattern of expression, consistent with their proposed functions as internal regulators. In addition, genome sequencing of the M. trichosporium OB3b PP358 mutant provides new evidence for the involvement of genes adjacent to the pMMO operon in methanotroph copper homeostasis.

XoxF Acts as the Predominant Methanol Dehydrogenase in the Type I Methanotroph Methylomicrobium buryatense

Many methylotrophic taxa harbor two distinct methanol dehydrogenase (MDH) systems for oxidizing methanol to formaldehyde: the well-studied calcium-dependent MxaFI type and the more recently discovered lanthanide-containing XoxF type. MxaFI has traditionally been accepted as the major functional MDH in bacteria that contain both enzymes. However, in this study, we present evidence that, in a type I methanotroph, Methylomicrobium buryatense, XoxF is likely the primary functional MDH in the environment. The addition of lanthanides increases xoxF expression and greatly reduces mxa expression, even under conditions in which calcium concentrations are almost 100-fold higher than lanthanide concentrations. Mutations in genes encoding the MDH enzymes validate our finding that XoxF is the major functional MDH, as XoxF mutants grow more poorly than MxaFI mutants under unfavorable culturing conditions. In addition, mutant and transcriptional analyses demonstrate that the lanthanide-dependent MDH switch operating in methanotrophs is mediated in part by the orphan response regulator MxaB, whose gene transcription is itself lanthanide responsive.

IMPORTANCE

Aerobic methanotrophs, bacteria that oxidize methane for carbon and energy, require a methanol dehydrogenase enzyme to convert methanol into formaldehyde. The calcium-dependent enzyme MxaFI has been thought to primarily carry out methanol oxidation in methanotrophs. Recently, it was discovered that XoxF, a lanthanide-containing enzyme present in most methanotrophs, can also oxidize methanol. In a methanotroph with both MxaFI and XoxF, we demonstrate that lanthanides transcriptionally control genes encoding the two methanol dehydrogenases, in part by controlling expression of the response regulator MxaB. Lanthanides are abundant in the Earth's crust, and we demonstrate that micromolar amounts of lanthanides are sufficient to suppress MxaFI expression. Thus, we present evidence that XoxF acts as the predominant methanol dehydrogenase in a methanotroph.

Advances and applications of molecular cloning in clinical microbiology

Molecular cloning is based on isolation of a DNA sequence of interest to obtain multiple copies of it in vitro. Application of this technique has become an increasingly important tool in clinical microbiology due to its simplicity, cost effectiveness, rapidity, and reliability. This review entails the recent advances in molecular cloning and its application in the clinical microbiology in the context of polymicrobial infections, recombinant antigens, recombinant vaccines, diagnostic probes, antimicrobial peptides, and recombinant cytokines. Culture-based methods in polymicrobial infection have many limitation, which has been overcome by cloning techniques and provide gold standard technique. Recombinant antigens produced by cloning technique are now being used for screening of HIV, HCV, HBV, CMV, Treponema pallidum, and other clinical infectious agents. Recombinant vaccines for hepatitis B, cholera, influenza A, and other diseases also use recombinant antigens which have replaced the use of live vaccines and thus reduce the risk for adverse effects. Gene probes developed by gene cloning have many applications including in early diagnosis of hereditary diseases, forensic investigations, and routine diagnosis. Industrial application of this technology produces new antibiotics in the form of antimicrobial peptides and recombinant cytokines that can be used as therapeutic agents.

Metabolic engineering in methanotrophic bacteria

Methane, as natural gas or biogas, is the least expensive source of carbon for (bio)chemical synthesis. Scalable biological upgrading of this simple alkane to chemicals and fuels can bring new sustainable solutions to a number of industries with large environmental footprints, such as natural gas/petroleum production, landfills, wastewater treatment, and livestock. Microbial biocatalysis with methane as a feedstock has been pursued off and on for almost a half century, with little enduring success. Today, biological engineering and systems biology provide new opportunities for metabolic system modulation and give new optimism to the concept of a methane-based bio-industry. Here we present an overview of the most recent advances pertaining to metabolic engineering of microbial methane utilization. Some ideas concerning metabolic improvements for production of acetyl-CoA and pyruvate, two main precursors for bioconversion, are presented. We also discuss main gaps in the current knowledge of aerobic methane utilization, which must be solved in order to release the full potential of methane-based biosystems.

Genetic tools for the industrially promising methanotroph Methylomicrobium buryatense

Aerobic methanotrophs oxidize methane at ambient temperatures and pressures and are therefore attractive systems for methane-based bioconversions. In this work, we developed and validated genetic tools for Methylomicrobium buryatense, a haloalkaliphilic gammaproteobacterial (type I) methanotroph. M. buryatense was isolated directly on natural gas and grows robustly in pure culture with a 3-h doubling time, enabling rapid genetic manipulation compared to many other methanotrophic species. As a proof of concept, we used a sucrose counterselection system to eliminate glycogen production in M. buryatense by constructing unmarked deletions in two redundant glycogen synthase genes. We also selected for a more genetically tractable variant strain that can be conjugated with small incompatibility group P (IncP)-based broad-host-range vectors and determined that this capability is due to loss of the native plasmid. These tools make M. buryatense a promising model system for studying aerobic methanotroph physiology and enable metabolic engineering in this bacterium for industrial biocatalysis of methane.

Perspective: what is known, and not known, about the connections between alkane oxidation and metal uptake in alkanotrophs in the marine environment

Should iron and copper be added to the environment to stimulate the natural bioremediation of marine oil spills? The key enzymes that catalyze the oxidation of alkanes require either iron or copper, and the concentration of these ions in seawater is vanishingly low. Nevertheless, the dependence of alkane oxidation activity on external metal concentrations remains unclear. This perspective will summarize what is known about the co-regulation of alkane oxidation and metal acquisition and pose a series of critical questions to which, for the most part, we do not yet have answers. The paucity of answers points to the need for additional studies to illuminate the cellular biology connecting microbial growth on alkanes to the acquisition of metal ions.

Bioconversion of natural gas to liquid fuel: opportunities and challenges

Natural gas is a mixture of low molecular weight hydrocarbon gases that can be generated from either fossil or anthropogenic resources. Although natural gas is used as a transportation fuel, constraints in storage, relatively low energy content (MJ/L), and delivery have limited widespread adoption. Advanced utilization of natural gas has been explored for biofuel production by microorganisms. In recent years, the aerobic bioconversion of natural gas (or primarily the methane content of natural gas) into liquid fuels (Bio-GTL) by biocatalysts (methanotrophs) has gained increasing attention as a promising alternative for drop-in biofuel production. Methanotrophic bacteria are capable of converting methane into microbial lipids, which can in turn be converted into renewable diesel via a hydrotreating process. In this paper, biodiversity, catalytic properties and key enzymes and pathways of these microbes are summarized. Bioprocess technologies are discussed based upon existing literature, including cultivation conditions, fermentation modes, bioreactor design, and lipid extraction and upgrading. This review also outlines the potential of Bio-GTL using methane as an alternative carbon source as well as the major challenges and future research needs of microbial lipid accumulation derived from methane, key performance index, and techno-economic analysis. An analysis of raw material costs suggests that methane-derived diesel fuel has the potential to be competitive with petroleum-derived diesel.

Architecture and active site of particulate methane monooxygenase

Particulate methane monooxygenase (pMMO) is an integral membrane metalloenzyme that oxidizes methane to methanol in methanotrophic bacteria, organisms that live on methane gas as their sole carbon source. Understanding pMMO function has important implications for bioremediation applications and for the development of new, environmentally friendly catalysts for the direct conversion of methane to methanol. Crystal structures of pMMOs from three different methanotrophs reveal a trimeric architecture, consisting of three copies each of the pmoB, pmoA, and pmoC subunits. There are three distinct metal centers in each protomer of the trimer, mononuclear and dinuclear copper sites in the periplasmic regions of pmoB and a mononuclear site within the membrane that can be occupied by copper or zinc. Various models for the pMMO active site have been proposed within these structural constraints, including dicopper, tricopper, and diiron centers. Biochemical and spectroscopic data on pMMO and recombinant soluble fragments, denoted spmoB proteins, indicate that the active site involves copper and is located at the site of the dicopper center in the pmoB subunit. Initial spectroscopic evidence for O(2) binding at this site has been obtained. Despite these findings, questions remain about the active site identity and nuclearity and will be the focus of future studies.

Genetic systems for moderately halo(alkali)philic bacteria of the genus Methylomicrobium

Biotechnologies for effective conversion of atmospheric greenhouse gases (CO(2) and CH(4)) into valuable compounds, such as chemical and petrochemical feedstocks or alternative fuels, offer promising new strategies for stabilization of global warming. A novel approach in this field involves the use of methanotrophic bacteria as catalysts for CH(4) conversion. In recent years, extremophilic methanotrophic species related to the genus Methylomicrobium have become favorable systems for bioprocess engineering, due to their high growth rates and tolerance of a wide range of environmental conditions and perturbations. While the cultures hold the potential of producing a broader range of chemicals from methane, the biotechnologies are still limited by the lack of reliable genetic approaches for system-level studies and strain engineering. In this chapter, we describe a set of molecular tools for genetic investigation and alteration of the Methylomicrobium spp.