Type IV-Like Pili Facilitate Transformation in Naturally Competent Archaea

Teaching old dogs new tricks: genetic engineering methanogens

Methanogenic archaea, which are integral to global carbon and nitrogen cycling, currently face challenges in genetic manipulation due to unique physiology and limited genetic tools. This review provides a survey of current and past developments in the genetic engineering of methanogens, including selection and counterselection markers, reporter systems, shuttle vectors, mutagenesis methods, markerless genetic exchange, and gene expression control. This review discusses genetic tools and emphasizes challenges tied to tool scarcity for specific methanogenic species. Mutagenesis techniques for methanogens, including physicochemical, transposon-mediated, liposome-mediated mutagenesis, and natural transformation, are outlined, along with achievements and challenges. Markerless genetic exchange strategies, such as homologous recombination and CRISPR/Cas-mediated genome editing, are also detailed. Finally, the review concludes by examining the control of gene expression in methanogens. The information presented underscores the urgent need for refined genetic tools in archaeal research. Despite historical challenges, recent advancements, notably CRISPR-based systems, hold promise for overcoming obstacles, with implications for global health, agriculture, climate change, and environmental engineering. This comprehensive review aims to bridge existing gaps in the literature, guiding future research in the expanding field of archaeal genetic engineering.

Functionally redundant formate dehydrogenases enable formate-dependent growth in Methanococcus maripaludis

Methanogens are essential for the complete remineralization of organic matter in anoxic environments. Most cultured methanogens are hydrogenotrophic, using H2 as an electron donor to reduce CO2 to CH4, but in the absence of H2 many can also use formate. Formate dehydrogenase (Fdh) is essential for formate oxidation, where it transfers electrons for the reduction of coenzyme F420 or to a flavin-based electron bifurcating reaction catalyzed by heterodisulfide reductase (Hdr), the terminal reaction of methanogenesis. Furthermore, methanogens that use formate encode at least two isoforms of Fdh in their genomes, but how these different isoforms participate in methanogenesis is unknown. Using Methanococcus maripaludis, we undertook a biochemical characterization of both Fdh isoforms involved in methanogenesis. Both Fdh1 and Fdh2 interacted with Hdr to catalyze the flavin-based electron bifurcating reaction, and both reduced F420 at similar rates. F420 reduction preceded flavin-based electron bifurcation activity for both enzymes. In a Δfdh1 mutant background, a suppressor mutation was required for Fdh2 activity. Genome sequencing revealed that this mutation resulted in the loss of a specific molybdopterin transferase (moeA), allowing for Fdh2-dependent growth, and the metal content of the proteins suggested that isoforms are dependent on either molybdenum or tungsten for activity. These data suggest that both isoforms of Fdh are functionally redundant, but their activities in vivo may be limited by gene regulation or metal availability under different growth conditions. Together these results expand our understanding of formate oxidation and the role of Fdh in methanogenesis.

Model Organisms To Study Methanogenesis, a Uniquely Archaeal Metabolism

Methanogenic archaea are the only organisms that produce CH4 as part of their energy-generating metabolism. They are ubiquitous in oxidant-depleted, anoxic environments such as aquatic sediments, anaerobic digesters, inundated agricultural fields, the rumen of cattle, and the hindgut of termites, where they catalyze the terminal reactions in the degradation of organic matter. Methanogenesis is the only metabolism that is restricted to members of the domain Archaea. Here, we discuss the importance of model organisms in the history of methanogen research, including their role in the discovery of the archaea and in the biochemical and genetic characterization of methanogenesis. We also discuss outstanding questions in the field and newly emerging model systems that will expand our understanding of this uniquely archaeal metabolism.

Random transposon mutagenesis identifies genes essential for transformation in Methanococcus maripaludis

Natural transformation, the process whereby a cell acquires DNA directly from the environment, is an important driver of evolution in microbial populations, yet the mechanism of DNA uptake is only characterized in bacteria. To expand our understanding of natural transformation in archaea, we undertook a genetic approach to identify a catalog of genes necessary for transformation in Methanococcus maripaludis. Using an optimized method to generate random transposon mutants, we screened 6144 mutant strains for defects in natural transformation and identified 25 transformation-associated candidate genes. Among these are genes encoding components of the type IV-like pilus, transcription/translation associated genes, genes encoding putative membrane bound transport proteins, and genes of unknown function. Interestingly, similar genes were identified regardless of whether replicating or integrating plasmids were provided as a substrate for transformation. Using allelic replacement mutagenesis, we confirmed that several genes identified in these screens are essential for transformation. Finally, we identified a homolog of a membrane bound substrate transporter in Methanoculleus thermophilus and verified its importance for transformation using allelic replacement mutagenesis, suggesting a conserved mechanism for DNA transfer in multiple archaea. These data represent an initial characterization of the genes important for transformation which will inform efforts to understand gene flow in natural populations. Additionally, knowledge of the genes necessary for natural transformation may assist in identifying signatures of transformation machinery in archaeal genomes and aid the establishment of new model genetic systems for studying archaea.

Interspecies Formate Exchange Drives Syntrophic Growth of Syntrophotalea carbinolica and Methanococcus maripaludis

The complete remineralization of organic matter in anoxic environments relies on communities of microorganisms that ferment organic acids and alcohols to CH₄. This is accomplished through syntrophic association of H₂ or formate producing bacteria and methanogenic archaea, where exchange of these intermediates enables growth of both organisms. While these communities are essential to Earth's carbon cycle, our understanding of the dynamics of H₂ or formate exchanged is limited. Here, we establish a model partnership between Syntrophotalea carbinolica and Methanococcus maripaludis. Through sequencing a transposon mutant library of M. maripaludis grown with ethanol oxidizing S. carbinolica, we found that genes encoding the F₄₂₀-dependent formate dehydrogenase (Fdh) and F₄₂₀-dependent methylene-tetrahydromethanopterin dehydrogenase (Mtd) are important for growth. Competitive growth of M. maripaludis mutants defective in either H₂ or formate metabolism verified that, across multiple substrates, interspecies formate exchange was dominant in these communities. Agitation of these cultures to facilitate diffusive loss of H₂ to the culture headspace resulted in an even greater competitive advantage for M. maripaludis strains capable of oxidizing formate. Finally, we verified that an M. maripaludis Δmtd mutant had a defect during syntrophic growth. Together, these results highlight the importance of formate exchange for the growth of methanogens under syntrophic conditions. IMPORTANCE In the environment, methane is typically generated by fermentative bacteria and methanogenic archaea working together in a process called syntrophy. Efficient exchange of small molecules like H₂ or formate is essential for growth of both organisms. However, difficulties in determining the relative contribution of these intermediates to methanogenesis often hamper efforts to understand syntrophic interactions. Here, we establish a model syntrophic coculture composed of S. carbinolica and the genetically tractable methanogen M. maripaludis. Using mutant strains of M. maripaludis that are defective for either H₂ or formate metabolism, we determined that interspecies formate exchange drives syntrophic growth of these organisms. Together, these results advance our understanding of the degradation of organic matter in anoxic environments.

Efficient CRISPR/Cas12a-Based Genome-Editing Toolbox for Metabolic Engineering in Methanococcus maripaludis

The rapid-growing and genetically tractable methanogen Methanococcus maripaludis is a promising host organism for the biotechnological conversion of carbon dioxide and renewable hydrogen to fuels and value-added products. Expansion of its product scope through metabolic engineering necessitates reliable and efficient genetic tools, particularly for genome edits that affect the primary metabolism and cell growth. Here, we have designed a genome-editing toolbox by utilizing Cas12a from Lachnospiraceae bacterium ND2006 (LbCas12a) in combination with the homology-directed repair machinery endogenously present in M. maripaludis. This toolbox can delete target genes with a success rate of up to 95%, despite the hyperpolyploidy of M. maripaludis. For the purpose of demonstrating a large deletion, the M. maripaludis flagellum operon (∼8.9 kbp) was replaced by the Escherichia coli β-glucuronidase gene. To facilitate metabolic engineering and flux balancing in M. maripaludis, the relative strength of 15 different promoters was quantified in the presence of two common growth substrates, either formate or carbon dioxide and hydrogen. This CRISPR/LbCas12a toolbox can be regarded as a reliable and quick method for genome editing in a methanogen.

The Fluorescence-Activating and Absorption-Shifting Tag (FAST) Enables Live-Cell Fluorescence Imaging of Methanococcus maripaludis

Live-cell fluorescence imaging of methanogenic archaea has been limited due to the strictly anoxic conditions required for growth and issues with autofluorescence associated with electron carriers in central metabolism. Here, we show that the fluorescence-activating and absorption-shifting tag (FAST) complexed with the fluorogenic ligand 4-hydroxy-3-methylbenzylidene-rhodanine (HMBR) overcomes these issues and displays robust fluorescence in Methanococcus maripaludis. We also describe a mechanism to visualize cells under anoxic conditions using a fluorescence microscope. Derivatives of FAST were successfully applied for protein abundance analysis, subcellular localization analysis, and determination of protein-protein interactions. FAST fusions to both formate dehydrogenase (Fdh) and F₄₂₀-reducing hydrogenase (Fru) displayed increased fluorescence in cells grown on formate-containing medium, consistent with previous studies suggesting the increased abundance of these proteins in the absence of H₂. Additionally, FAST fusions to both Fru and the ATPase associated with the archaellum (FlaI) showed a membrane localization in single cells observed using anoxic fluorescence microscopy. Finally, a split reporter translationally fused to the alpha and beta subunits of Fdh reconstituted a functionally fluorescent molecule in vivo via bimolecular fluorescence complementation. Together, these observations demonstrate the utility of FAST as a tool for studying members of the methanogenic archaea. IMPORTANCE Methanogenic archaea are important members of anaerobic microbial communities where they catalyze essential reactions in the degradation of organic matter. Developing additional tools for studying the cell biology of these organisms is essential to understanding them at a mechanistic level. Here, we show that FAST, in combination with the fluorogenic ligand HMBR, can be used to monitor protein dynamics in live cells of M. maripaludis. The application of FAST holds promise for future studies focused on the metabolism and physiology of methanogenic archaea.

Horizontal Gene Transfer in Archaea-From Mechanisms to Genome Evolution

Archaea remains the least-studied and least-characterized domain of life despite its significance not just to the ecology of our planet but also to the evolution of eukaryotes. It is therefore unsurprising that research into horizontal gene transfer (HGT) in archaea has lagged behind that of bacteria. Indeed, several archaeal lineages may owe their very existence to large-scale HGT events, and thus understanding both the molecular mechanisms and the evolutionary impact of HGT in archaea is highly important. Furthermore, some mechanisms of gene exchange, such as plasmids that transmit themselves via membrane vesicles and the formation of cytoplasmic bridges that allows transfer of both chromosomal and plasmid DNA, may be archaea specific. This review summarizes what we know about HGT in archaea, and the barriers that restrict it, highlighting exciting recent discoveries and pointing out opportunities for future research. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Overview of Diverse Methyl/Alkyl-Coenzyme M Reductases and Considerations for Their Potential Heterologous Expression

Methyl-coenzyme M reductase (MCR) is an archaeal enzyme that catalyzes the final step of methanogenesis and the first step in the anaerobic oxidation of methane, the energy metabolisms of methanogens and anaerobic methanotrophs (ANME), respectively. Variants of MCR, known as alkyl-coenzyme M reductases, are involved in the anaerobic oxidation of short-chain alkanes including ethane, propane, and butane as well as the catabolism of long-chain alkanes from oil reservoirs. MCR is a dimer of heterotrimers (encoded by mcrABG) and requires the nickel-containing tetrapyrrole prosthetic group known as coenzyme F₄₃₀. MCR houses a series of unusual post-translational modifications within its active site whose identities vary depending on the organism and whose functions remain unclear. Methanogenic MCRs are encoded in a highly conserved mcrBDCGA gene cluster, which encodes two accessory proteins, McrD and McrC, that are believed to be involved in the assembly and activation of MCR, respectively. The requirement of a unique and complex coenzyme, various unusual post-translational modifications, and many remaining questions surrounding assembly and activation of MCR largely limit in vitro experiments to native enzymes with recombinant methods only recently appearing. Production of MCRs in a heterologous host is an important step toward developing optimized biocatalytic systems for methane production as well as for bioconversion of methane and other alkanes into value-added compounds. This review will first summarize MCR catalysis and structure, followed by a discussion of advances and challenges related to the production of diverse MCRs in a heterologous host.

A Shuttle-Vector System Allows Heterologous Gene Expression in the Thermophilic Methanogen Methanothermobacter thermautotrophicus ΔH

Thermophilic Methanothermobacter spp. are used as model microbes to study the physiology and biochemistry of the conversion of molecular hydrogen and carbon dioxide into methane (i.e., hydrogenotrophic methanogenesis). Yet, a genetic system for these model microbes was missing despite intensive work for four decades. Here, we report the successful implementation of genetic tools for Methanothermobacter thermautotrophicus ΔH. We developed shuttle vectors that replicated in Escherichia coli and M. thermautotrophicus ΔH. For M. thermautotrophicus ΔH, a thermostable neomycin resistance cassette served as the selectable marker for positive selection with neomycin, and the cryptic plasmid pME2001 from Methanothermobacter marburgensis served as the replicon. The shuttle-vector DNA was transferred from E. coli into M. thermautotrophicus ΔH via interdomain conjugation. After the successful validation of DNA transfer and positive selection in M. thermautotrophicus ΔH, we demonstrated heterologous gene expression of a thermostable β-galactosidase-encoding gene (bgaB) from Geobacillus stearothermophilus under the expression control of four distinct synthetic and native promoters. In quantitative in-vitro enzyme activity assay, we found significantly different β-galactosidase activity with these distinct promoters. With a formate dehydrogenase operon-encoding shuttle vector, we allowed growth of M. thermautotrophicus ΔH on formate as the sole growth substrate, while this was not possible for the empty-vector control.

The Oligosaccharyltransferase AglB Supports Surface-Associated Growth and Iron Oxidation in Methanococcus maripaludis

Most microbial organisms grow as surface-attached communities known as biofilms. However, the mechanisms whereby methanogenic archaea grow attached to surfaces have remained understudied. Here, we show that the oligosaccharyltransferase AglB is essential for growth of Methanococcus maripaludis strain JJ on glass or metal surfaces. AglB glycosylates several cellular structures, such as pili, archaella, and the cell surface layer (S-layer). We show that the S-layer of strain JJ, but not strain S2, is a glycoprotein, that only strain JJ was capable of growth on surfaces, and that deletion of aglB blocked S-layer glycosylation and abolished surface-associated growth. A strain JJ mutant lacking structural components of the type IV-like pilus did not have a growth defect under any conditions tested, while a mutant lacking the preflagellin peptidase (ΔflaK) was defective for surface growth only when formate was provided as the sole electron donor. Finally, for strains that are capable of Fe⁰ oxidation, we show that deletion of aglB decreases the rate of anaerobic Fe⁰ oxidation, presumably due to decreased association of biomass with the Fe⁰ surface. Together, these data provide an initial characterization of surface-associated growth in a member of the methanogenic archaea. IMPORTANCE Methanogenic archaea are responsible for producing the majority of methane on Earth and catalyze the terminal reactions in the degradation of organic matter in anoxic environments. Methanogens often grow as biofilms associated with surfaces or partner organisms; however, the molecular details of surface-associated growth remain uncharacterized. We have found evidence that glycosylation of the cell surface layer is essential for growth of M. maripaludis on surfaces and can enhance rates of anaerobic iron corrosion. These results provide insight into the physiology of surface-associated methanogenic organisms and highlight the importance of surface association for anaerobic iron corrosion.

Formate-Dependent Heterodisulfide Reduction in a Methanomicrobiales Archaeon

Hydrogenotrophic methanogens produce CH₄ using H₂ as an electron donor to reduce CO₂ In the absence of H₂, many are able to use formate or alcohols as alternate electron donors. Methanogens from the order Methanomicrobiales are capable of growth with H₂, but many lack genes encoding hydrogenases that are typically found in other hydrogenotrophic methanogens. In an effort to better understand electron flow in methanogens from the Methanomicrobiales, we undertook a genetic and biochemical study of heterodisulfide reductase (Hdr) in Methanoculleus thermophilus Hdr catalyzes an essential reaction by coupling the first and last steps of methanogenesis through flavin-based electron bifurcation. Hdr from M. thermophilus copurified with formate dehydrogenase (Fdh) and only displayed activity when formate was supplied as an electron donor. We found no evidence of an Hdr-associated hydrogenase, and H₂ could not function as an electron donor, even with Hdr purified from cells grown on H₂ We found that cells catalyze a formate hydrogenlyase activity that is likely essential for generating the formate needed for the Hdr reaction. Together, these results highlight the importance of formate as an electron donor for methanogenesis and suggest the ability to use formate is closely integrated into the methanogenic pathway in organisms from the order Methanomicrobiales IMPORTANCE Methanogens from the order Methanomicrobiales are thought to prefer H₂ as an electron donor for growth. They are ubiquitous in anaerobic environments, such as in wastewater treatment facilities, anaerobic digesters, and the rumen, where they catalyze the terminal steps in the breakdown of organic matter. However, despite their importance, the metabolism of these organisms remains understudied. Using a genetic and biochemical approach, we show that formate metabolism is closely integrated into methanogenesis in Methanoculleus thermophilus This is due to a requirement for formate as the electron donor to heterodisulfide reductase (Hdr), an enzyme responsible for catalyzing essential reactions in methanogenesis by linking the initial CO₂ fixing step to the exergonic terminal reaction of the pathway. These results suggest that hydrogen is not necessarily the preferred electron donor for all hydrogenotrophic methanogens and provide insight into the metabolism of methanogens from the order Methanomicrobiales.

Transcriptional regulation of methanogenic metabolism in archaea

Methanogenesis is a widespread metabolism of evolutionary and environmental importance that is likely to have originated on early Earth. Microorganisms that perform methanogenesis, termed methanogens, belong exclusively to the domain Archaea. Despite maintaining eukaryotic transcription machinery and homologs of bacterial regulators, archaeal transcription and gene regulation appear to be distinct from either domain. While genes involved in methanogenic metabolism have been identified and characterized, their regulation in response to both extracellular and intracellular signals is less understood. Here, we review recent reports on transcriptional regulation of methanogenesis using two model methanogens, Methanococcus maripaludis and Methanosarcina acetivorans, and highlight directions for future research in this nascent field.