Identification of an upstream regulatory sequence that mediates the transcription of mox genes in Methylobacterium extorquens AM1

Expression, purification and testing of lanthanide-dependent enzymes in Methylorubrum extorquens AM1

With mounting evidence of the importance of lanthanide metals in biology and among diverse bacterial phyla, a platform for high-throughput microbial growth for expression and purification of lanthanide-dependent enzymes is increasingly important. Presented in this chapter is a stream-lined approach for growth of the model methylotrophic bacterium Methylorubrum extorquens AM1 for the expression of lanthanide-dependent enzymes. Growth is optimized for both high-throughput phenotypic characterization facilitating in vivo studies, as well as for scaled-up batch cultivation for enzyme purification allowing for in vitro enzymatic studies. Both approaches have been shown to be important to understanding the function and structure of these enzymes. Expression systems have been designed for production of enzymes with and without lanthanide metals, allowing for detection of lanthanide dependence. The protocol described herein is expected to accelerate the discovery of novel lanthanide-dependent enzymes and our understanding of the role of these metals in the greater biological world.

Parallel and Divergent Evolutionary Solutions for the Optimization of an Engineered Central Metabolism in Methylobacterium extorquens AM1

Bioengineering holds great promise to provide fast and efficient biocatalysts for methanol-based biotechnology, but necessitates proven methods to optimize physiology in engineered strains. Here, we highlight experimental evolution as an effective means for optimizing an engineered Methylobacterium extorquens AM1. Replacement of the native formaldehyde oxidation pathway with a functional analog substantially decreased growth in an engineered Methylobacterium, but growth rapidly recovered after six hundred generations of evolution on methanol. We used whole-genome sequencing to identify the basis of adaptation in eight replicate evolved strains, and examined genomic changes in light of other growth and physiological data. We observed great variety in the numbers and types of mutations that occurred, including instances of parallel mutations at targets that may have been "rationalized" by the bioengineer, plus other "illogical" mutations that demonstrate the ability of evolution to expose unforeseen optimization solutions. Notably, we investigated mutations to RNA polymerase, which provided a massive growth benefit but are linked to highly aberrant transcriptional profiles. Overall, we highlight the power of experimental evolution to present genetic and physiological solutions for strain optimization, particularly in systems where the challenges of engineering are too many or too difficult to overcome via traditional engineering methods.

Ethylmalonyl coenzyme A mutase operates as a metabolic control point in Methylobacterium extorquens AM1

The metabolism of one- and two-carbon compounds by the methylotrophic bacterium Methylobacterium extorquens AM1 involves high carbon flux through the ethylmalonyl coenzyme A (ethylmalonyl-CoA) pathway (EMC pathway). During growth on ethylamine, the EMC pathway operates as a linear pathway carrying the full assimilatory flux to produce glyoxylate, malate, and succinate. Assimilatory carbon enters the ethylmalonyl-CoA pathway directly as acetyl-CoA, bypassing pathways for formaldehyde oxidation/assimilation and the regulatory mechanisms controlling them, making ethylamine growth a useful condition to study the regulation of the EMC pathway. Wild-type M. extorquens cells were grown at steady state on a limiting concentration of succinate, and the growth substrate was then switched to ethylamine, a condition where the cell must make a sudden switch from utilizing the tricarboxylic acid (TCA) cycle to using the ethylmalonyl-CoA pathway for assimilation, which has been an effective strategy for identifying metabolic control points. A 9-h lag in growth was observed, during which butyryl-CoA, a degradation product of ethylmalonyl-CoA, accumulated, suggesting a metabolic imbalance. Ethylmalonyl-CoA mutase activity increased to a level sufficient for the observed growth rate at 9 h, which correlated with an upregulation of RNA transcripts for ecm and a decrease in the levels of ethylmalonyl-CoA. When the wild-type strain overexpressing ecm was tested with the same substrate switchover experiment, ethylmalonyl-CoA did not accumulate, growth resumed earlier, and, after a transient period of slow growth, the culture grew at a higher rate than that of the control. These findings demonstrate that ethylmalonyl-CoA mutase is a metabolic control point in the EMC pathway, expanding our understanding of its regulation.

Methylobacterium extorquens: methylotrophy and biotechnological applications

Methylotrophy is the ability to use reduced one-carbon compounds, such as methanol, as a single source of carbon and energy. Methanol is, due to its availability and potential for production from renewable resources, a valuable feedstock for biotechnology. Nature offers a variety of methylotrophic microorganisms that differ in their metabolism and represent resources for engineering of value-added products from methanol. The most extensively studied methylotroph is the Alphaproteobacterium Methylobacterium extorquens. Over the past five decades, the metabolism of M. extorquens has been investigated physiologically, biochemically, and more recently, using complementary omics technologies such as transcriptomics, proteomics, metabolomics, and fluxomics. These approaches, together with a genome-scale metabolic model, facilitate system-wide studies and the development of rational strategies for the successful generation of desired products from methanol. This review summarizes the knowledge of methylotrophy in M. extorquens, as well as the available tools and biotechnological applications.

Metabolic engineering of Methylobacterium extorquens AM1 for 1-butanol production

BACKGROUND

Butanol is a promising next generation fuel and a bulk chemical precursor. Although clostridia are the primary industrial microbes for the fermentative production of 1-butanol, alternative engineered hosts have the potential to generate 1-butanol from alternative carbon feedstocks via synthetic metabolic pathways. Methylobacterium extorquens AM1, a facultative methylotrophic α-proteobacterium, is a model system for assessing the possibility of generating products such as 1-butanol from one-carbon and two-carbon feedstocks. Moreover, the core methylotrophic pathways in M. extorquens AM1 involve unusual coenzyme A (CoA)-derivative metabolites, such as crotonyl-CoA, which is a precursor for the production of 1-butanol.

RESULTS

In this work, we engineered a modified CoA-dependent pathway in Methylobacterium extorquens AM1 to produce 1-butanol. Engineered strains displayed different 1-butanol titers using ethylamine as a substrate. A strain overexpressing Treponema denticola trans-enoyl-CoA reductase, Clostridium acetobutylicum alcohol dehydrogenase, and native crotonase was able to generate the highest 1-butanol titer (15.2 mg l(-1)). In vitro isotopic tracing of metabolic flux and in vivo metabolite analysis showed the accumulation of butyryl-CoA, demonstrating the functionality of the synthetic pathway and identifying targets for future improvement.

CONCLUSIONS

We demonstrated the feasibility of using metabolic intermediates of the ethylmalonyl-CoA pathway in M. extorquens AM1 to generate value-added chemicals, with 1-butanol as the test case. This will not only establish the biotechnological potential of the ethylmalonyl-CoA pathway, but will also introduce M. extorquens AM1 as a potential platform to produce value-added chemicals.

A novel pair of inducible expression vectors for use in Methylobacterium extorquens

BACKGROUND

Due to the ever increasing use of diverse microbial taxa in basic research and industrial settings, there is a growing need for genetic tools to alter the physiology of these organisms. In particular, there is a dearth of inducible expression systems available for bacteria outside commonly used γ-proteobacteria, such as Escherichia coli or Pseudomonas species. To this end, we have sought to develop a pair of inducible expression vectors for use in the α-proteobacterium Methylobacterium extorquens, a model methylotroph.

FINDINGS

We found that the P(R) promoter from rhizobial phage 16-3 was active in M. extorquens and engineered the promoter to be inducible by either p-isopropyl benzoate (cumate) or anhydrotetracycline. These hybrid promoters, P(R/cmtO) and P(R/tetO), were found to have high levels of expression in M. extorquens with a regulatory range of 10-fold and 30-fold, respectively. Compared to an existing cumate-inducible (10-fold range), high-level expression system for M. extorquens, P(R/cmtO) and P(R/tetO) have 33% of the maximal activity but were able to repress gene expression 3 and 8-fold greater, respectively. Both promoters were observed to exhibit homogeneous, titratable activation dynamics rather than on-off, switch-like behavior. The utility of these promoters was further demonstrated by complementing loss of function of ftfL--essential for growth on methanol--where we show P(R/tetO) is capable of not only fully complementing function but also producing a conditional null phenotype. These promoters have been incorporated into a broad-host-range backbone allowing for potential use in a variety of bacterial hosts.

CONCLUSIONS

We have developed two novel expression systems for use in M. extorquens. The expression range of these vectors should allow for increased ability to explore cellular physiology in M. extorquens. Further, the P(R/tetO) promoter is capable of producing conditional null phenotypes, previously unattainable in M. extorquens. As both expression systems rely on the use of membrane permeable inducers, we suspect these expression vectors will be useful for ectopic gene expression in numerous proteobacteria.

MdoR is a novel positive transcriptional regulator for the oxidation of methanol in Mycobacterium sp. strain JC1

Mycobacterium sp. strain JC1 is able to grow on methanol as a sole source of carbon and energy using methanol:N,N'-dimethyl-4-nitrosoaniline oxidoreductase (MDO) as a key enzyme for methanol oxidation. The second open reading frame (mdoR) upstream of, and running divergently from, the mdo gene was identified as a gene for a TetR family transcriptional regulator. The N-terminal region of MdoR contained a helix-turn-helix DNA-binding motif. An electrophoretic mobility shift assay (EMSA) indicated that MdoR could bind to a mdo promoter region containing an inverted repeat. The mdoR deletion mutant did not grow on methanol, but growth on methanol was restored by a plasmid containing an intact mdoR gene. In DNase I footprinting and EMSA experiments, MdoR bound to two inverted repeats in the putative mdoR promoter region. Reverse transcription-PCR indicated that the mdoR gene was transcribed only in cells growing on methanol, whereas β-galactosidase assays showed that the mdoR promoter was activated in the presence of methanol. These results indicate that MdoR serves as a transcriptional activator for the expression of mdo and its own gene. Also, MdoR is the first discovered member of the TetR family of transcriptional regulators to be involved in the regulation of the methanol oxidation, as well as to function as a positive autoregulator.

Comparative analysis of two types of methanol dehydrogenase from Methylophaga aminisulfidivorans MPT grown on methanol

Two types of methanol dehydrogenase (MDH) were obtained from a novel marine methylotrophic bacterium, Methylophaga aminisulfidivorans MP(T), grown on methanol. Type I MDH consisted of two identical dimers of α (65.98 kDa) and β (7.58 kDa) subunits organized to form the α(2)β(2) tetramer. Type II MDH contained an additional MxaJ protein (27.86 kDa) and had more specific activity than type I MDH. The K(m) values of type I and II MDH for methanol under cytochrome c(L) reduction assay system were estimated to be 50.3 and 13.0 μM, respectively, and the isoelectric points of type I and II MDH were determined to be 5.4 and 5.8, respectively. The average molar ratios of α:β, α:MxaJ, and β:MxaJ in type II MDH were approximately 1:0.99, 1:0.41 and 1:0.42, respectively. Based on these results, the original conformation of the MDH of M. aminisulfidivorans MP(T) is most likely the α(2)β(2)-MxaJ complex. During purification, the lysozyme and freeze-thawing cell disruption method significantly increased the amount of type II MDH in the soluble fraction compared with strong physical disruption methods such as sonication and French Press.

Fast growth increases the selective advantage of a mutation arising recurrently during evolution under metal limitation

Understanding the evolution of biological systems requires untangling the molecular mechanisms that connect genetic and environmental variations to their physiological consequences. Metal limitation across many environments, ranging from pathogens in the human body to phytoplankton in the oceans, imposes strong selection for improved metal acquisition systems. In this study, we uncovered the genetic and physiological basis of adaptation to metal limitation using experimental populations of Methylobacterium extorquens AM1 evolved in metal-deficient growth media. We identified a transposition mutation arising recurrently in 30 of 32 independent populations that utilized methanol as a carbon source, but not in any of the 8 that utilized only succinate. These parallel insertion events increased expression of a novel transporter system that enhanced cobalt uptake. Such ability ensured the production of vitamin B₁₂, a cobalt-containing cofactor, to sustain two vitamin B₁₂–dependent enzymatic reactions essential to methanol, but not succinate, metabolism. Interestingly, this mutation provided higher selective advantages under genetic backgrounds or incubation temperatures that permit faster growth, indicating growth-rate–dependent epistatic and genotype-by-environment interactions. Our results link beneficial mutations emerging in a metal-limiting environment to their physiological basis in carbon metabolism, suggest that certain molecular features may promote the emergence of parallel mutations, and indicate that the selective advantages of some mutations depend generically upon changes in growth rate that can stem from either genetic or environmental influences.

Effects of mutations can change under different genetic backgrounds or environmental factors, also known as epistasis and genotype-by-environment interactions (G×E), respectively. Though epistasis and G×E are traditionally treated as distinct phenomena, our study of a beneficial mutation highlights their commonality. This mutation resulted from insertion of the same transposable element upstream of a novel cobalt transport system in 30 of 32 independent populations during evolution in metal-limited media. The resulting increased cobalt uptake provided a selective benefit that depended upon two environmental factors: cobalt limitation and growth substrates whose metabolism requires a particular vitamin B₁₂ (which contains cobalt) -dependent biochemical pathway. Furthermore, this mutation exhibited epistatic and G×E interactions with other cellular processes in a generic way, such that its selective advantage increased as cells were able to grow faster. This growth-rate dependence accords with a simple model: the slowest of multiple physiological processes needed for growth exerts the greatest control over an organism's growth rate. It suggests that as growth results from the performance of the entire physiological system, genes or environmental factors that affect distinct physiological processes may thus interact through their convergent effects on growth phenotypes.

Asymmetric, bimodal trade-offs during adaptation of Methylobacterium to distinct growth substrates

Trade-offs between selected and nonselected environments are often assumed to exist during adaptation. This phenomenon is prevalent in microbial metabolism, where many organisms have come to specialize on a narrow breadth of substrates. One well-studied example is methylotrophic bacteria that can use single-carbon (C(1)) compounds as their sole source of carbon and energy, but generally use few, if any, multi-C compounds. Here, we use adaptation of experimental populations of the model methylotroph, Methylobacterium extorquens AM1, to C(1) (methanol) or multi-C (succinate) compounds to investigate specialization and trade-offs between these two metabolic lifestyles. We found a general trend toward trade-offs during adaptation to succinate, but this was neither universal nor showed a quantitative relationship with the extent of adaptation. After 1500 generations, succinate-evolved strains had a remarkably bimodal distribution of fitness values on methanol: either an improvement comparable to the strains adapted on methanol or the complete loss of the ability to grow on C(1) compounds. In contrast, adaptation to methanol resulted in no such trade-offs. Based on the substantial, asymmetric loss of C(1) growth during growth on succinate, we suggest that the long-term maintenance of C(1) metabolism across the genus Methylobacterium requires relatively frequent use of C(1) compounds to prevent rapid loss.

Implementation of microarrays for Methylobacterium extorquens AM1

Microarrays are an important tool for understanding global gene expression changes, and the resulting data sets can be used to direct physiologic and metabolic studies. To take advantage of this technology, 60-mer oligonucleotide microarrays were designed for Methylobacterium extorquens AM1 to study gene expression changes that occur under differing physiological conditions. The carbon utilization pathways for methanol and succinate have been well characterized, and growth with these substrates was chosen as the condition used to validate the microarray data. The data were analyzed using two different methods and compared to previously obtained experimental data. The array data processed using the Significance Analysis of Microarrays followed by p-value assessment, correlated best to the experimental data. In addition to validating the microarrays, these studies uncovered possible connections between methylotrophy, iron, and sulfur homeostasis, bacteriochlorophyll production and polyketide synthesis, and will likely aid in uncovering further metabolic networks and genes required for methylotrophy.