The Role of Serine-Glyoxylate Aminotransferase and Malyl-CoA Lyase in the Metabolism of Methylococcus capsulatus Bath.

The genome of aerobic methanotroph Methylococcus capsulatus Bath possesses genes of three biochemical pathways of C1-carbon assimilation: the ribulose monophosphate cycle, the Calvin-Benson-Bassham cycle, and the partial serine cycle. Numerous studies have demonstrated that during methanotrophic growth cells of Methylococcus capsulatus Bath express key enzymes of these routes. In this study, the role of the serine cycle key enzymes, serine-glyoxylate aminotransferase (Sga) and malyl-CoA lyase (Mcl) in metabolism of Methylococcus capsulatus Bath was investigated by gene inactivation. The Δsga mutant obtained by double homologous recombination showed a prolonged lag phase, and after the lag period, the growth rate became similar to that of the wild type strain. The elevated intracellular levels of glutamate, serine, glycine, alanine, methionine, leucine, and succinate suggested significant metabolic changes in the mutant cells. Deletion of the mcl gene resulted in very poor growth and glycine only partially improved growth of the mutant strain. Cells of Δmcl mutant possess lower content of histidine, but enhanced level of alanine, leucine, and lysine than those of the wild type strain. Our data imply the importance of the serine cycle enzymes in metabolism of the methanotroph as well as relationships of the three C1 assimilation pathways in the gammaproteobacterial methanotrophs.

AcdR protein is an activator of transcription of 1-aminocyclopropane-1-carboxylate deaminase in Methylobacterium radiotolerans JCM 2831.

It has been previously shown that a number of plant associated methylotrophic bacteria contain an enzyme aminocyclopropane carboxylate (ACC) deaminase (AcdS) hydrolyzing ACC, the immediate precursor of ethylene in plants. The genome of the epiphytic methylotroph Methylobacterium radiotolerans JCM2831 contains an open reading frame encoding a protein homologous to transcriptional regulatory protein AcdR of the Lrp (leucine-responsive regulatory protein) family. The acdR gene of M. radiotolerans was heterologously expressed in Escherichia coli and purified. The results of gel retardation experiments have shown that AcdR specifically binds the DNA fragment containing the promoter-operator region of the acdS gene. ACC decreased electrophoretic mobility of the AcdR-DNA complex whereas leucine had no effect on the complex mobility. The mutant strains of M. radiotolerans obtained by insertion of a tetracycline cassette in the acdS or acdR gene lost the ACC-deaminase activity but the strains with complementation of the mutation recovered this function. The acdS- mutant but not acdR- strain expressed the xylE reporter gene under the control of acdS promoter region thus resulting in a catechol 2,3-dioxygenase activity. This suggested that AcdR in vivo functions as activator of transcription of the acdS gene. The results obtained in this study showed that in phytosymbiotic methylotroph Methylobacterium radiotolerans AcdR mediates activation of the acdS gene transcription in the presence of an inducer ACC or 2-aminoisobutyrate and the excess of the regulatory protein assists in transcription initiation even in the absence of the inducer. The model of regulation of acdS transcription in M. radiotolerans was proposed.

Alteration of cofactor specificity of the acrylyl-CoA reductase from Escherichia coli.

OBJECTIVES: Alteration of the cofactor specificity of acrylyl-CoA reductase (AcuI) catalyzing the NAD(P)H-dependent reduction of acrylyl-CoA to propionyl-CoA is often desirable for designing of artificial metabolic pathways of various appointments. RESULTS: Several variants of AcuIs from Escherichia coli K-12 with multiple amino acid substitutions to alter the cofactor preference were obtained by site directed mutagenesis and the modified enzymes as His6-tagged proteins were characterized. The simultaneous substitutions of arginine-180, arginine-198 and serine-178 residues by alanine in the enzyme pocket sequence as well as other amino acid changes decreased both NADPH- and NADH-dependent activities in comparison to the wild-type enzyme. The replacement of serine-156 by glutamic acid decreased NADPH-dependent activity at least 7000-fold but NADH-dependent activity only by threefold. The replacement of serine-156 by aspartic acid decreased NADPH-dependent activity 70-fold with fair preservation of activity and specificity to NADH. CONCLUSIONS: These results demonstrated a relevance of Asp156 in the interaction of AcuI from E. coli K-12 with NADH as a coenzyme. These findings may provide reference information for shifting coenzyme specificity of acrylyl-CoA reductases.

Metabolic Features of Aerobic Methanotrophs: News and Views.

This review is focused on recent studies of carbon metabolism in aerobic methanotrophs that specifically addressed the properties, distribution and phylogeny of some of the key enzymes involved in assimilation of carbon from methane. These include enzymes involved in sugar sythesis and cleavage, conversion of intermediates of the tricarboxylic acid cycle, as well as in osmoadaptation in halotolerant methanotrophs.

The genes and enzymes of sucrose metabolism in moderately thermophilic methanotroph Methylocaldum szegediense O12.

Four enzymes involved in sucrose metabolism: sucrose phosphate synthase (Sps), sucrose phosphate phosphatase (Spp), sucrose synthase (Sus) and fructokinase (FruK), were obtained as his-tagged proteins from the moderately thermophilic methanotroph Methylocaldum szegediense O12. Sps, Spp, FruK and Sus demonstrated biochemical properties similar to those of other bacterial counterparts, but the translated amino acid sequences of Sps and Spp displayed high divergence from the respective microbial enzymes. The Sus of M. szegediense O12 catalyzed the reversible reaction of sucrose cleavage in the presence of ADP or UDP and preferred ADP as a substrate, thus implying a connection between sucrose and glycogen metabolism. Sus-like genes were found only in a few methanotrophs, whereas amylosucrase was generally used in sucrose cleavage in this group of bacteria. Like other microbial fructokinases, FruK of M. szegediense O12 showed a high specificity to fructose.

The properties and potential metabolic role of glucokinase in halotolerant obligate methanotroph Methylomicrobium alcaliphilum 20Z.

Aerobic bacteria utilizing methane as the carbon and energy source do not use sugars as growth substrates but possess the gene coding for glucokinase (Glk), an enzyme converting glucose into glucose 6-phosphate. Here we demonstrate the functionality and properties of Glk from an obligate methanotroph Methylomicrobium alcaliphilum 20Z. The recombinant Glk obtained by heterologous expression in Escherichia coli was found to be close in biochemical properties to other prokaryotic Glks. The homodimeric enzyme (2 × 35 kDa) catalyzed ATP-dependent phosphorylation of glucose and glucosamine with nearly equal activity, being inhibited by ADP (K i = 2.34 mM) but not affected by glucose 6-phosphate. Chromosomal deletion of the glk gene resulted in a loss of Glk activity and retardation of growth as well as in a decrease of intracellular glycogen content. Inactivation of the genes encoding sucrose phosphate synthase or amylosucrase, the enzymes involved in glycogen biosynthesis via sucrose as intermediate, did not prevent glycogen accumulation. In silico analysis revealed glk orthologs predominantly in methanotrophs harboring glycogen synthase genes. The data obtained suggested that Glk is implicated in the regulation of glycogen biosynthesis/degradation in an obligate methanotroph.

Sucrose metabolism in halotolerant methanotroph Methylomicrobium alcaliphilum 20Z.

Sucrose accumulation has been observed in some methylotrophic bacteria utilizing methane, methanol, or methylated amines as a carbon and energy source. In this work, we have investigated the biochemical pathways for sucrose metabolism in the model halotolerant methanotroph Methylomicrobium alcaliphilum 20Z. The genes encoding sucrose-phosphate synthase (Sps), sucrose-phosphate phosphatase (Spp), fructokinase (FruK), and amylosucrase (Ams) were co-transcribed and displayed similar expression levels. Functional Spp and Ams were purified after heterologous expression in Escherichia coli. Recombinant Spp exhibited high affinity for sucrose-6-phosphate and stayed active at very high levels of sucrose (K i  = 1.0 ± 0.6 M). The recombinant amylosucrase obeyed the classical Michaelis-Menten kinetics in the reactions of sucrose hydrolysis and transglycosylation. As a result, the complete metabolic network for sucrose biosynthesis and re-utilization in the non-phototrophic organism was reconstructed for the first time. Comparative genomic studies revealed analogous gene clusters in various Proteobacteria, thus indicating that the ability to produce and metabolize sucrose is widespread among prokaryotes.

Acetate kinase-an enzyme of the postulated phosphoketolase pathway in Methylomicrobium alcaliphilum 20Z.

Recombinant acetate kinase (AcK) was obtained from the aerobic haloalkalitolerant methanotroph Methylomicrobium alcaliphilum 20Z by heterologous expression in Escherichia coli and purification by affinity chromatography. The substrate specificity, the kinetics and oligomeric state of the His6-tagged AcK were determined. The M. alcaliphilum AcK (2 × 45 kDa) catalyzed the reversible phosphorylation of acetate into acetyl phosphate and exhibited a dependence on Mg(2+) or Mn(2+) ions and strong specificity to ATP/ADP. The enzyme showed the maximal activity and high stability at 70 °C. AcK was 20-fold more active in the reaction of acetate synthesis compared to acetate phosphorylation and had a higher affinity to acetyl phosphate (K m 0.11 mM) than to acetate (K m 5.6 mM). The k cat /K m ratios indicated that the enzyme had a remarkably high catalytic efficiency for acetate and ATP formation (k cat/K m = 1.7 × 10(6)) compared to acetate phosphorylation (k cat/K m = 2.5 × 10(3)). The ack gene of M. alcaliphilum 20Z was shown to be co-transcribed with the xfp gene encoding putative phosphoketolase. The Blast analysis revealed the ack and xfp genes in most genomes of the sequenced aerobic methanotrophs, as well as methylotrophic bacteria not growing on methane. The distribution and metabolic role of the postulated phosphoketolase shunted glycolytic pathway in aerobic C1-utilizing bacteria is discussed.

Genomic Insights into Moderately Thermophilic Methanotrophs of the Genus Methylocaldum.

Considering the increasing interest in understanding the biotic component of methane removal from our atmosphere, it becomes essential to study the physiological characteristics and genomic potential of methanotroph isolates, especially their traits allowing them to adapt to elevated growth temperatures. The genetic signatures of Methylocaldum species have been detected in many terrestrial and aquatic ecosystems. A small set of representatives of this genus has been isolated and maintained in culture. The genus is commonly described as moderately thermophilic, with the growth optimum reaching 50 °C for some strains. Here, we present a comparative analysis of genomes of three Methylocaldum strains-two terrestrial M. szegediense strains (O-12 and Norfolk) and one marine strain, Methylocaldum marinum (S8). The examination of the core genome inventory of this genus uncovers significant redundancy in primary metabolic pathways, including the machinery for methane oxidation (numerous copies of pmo genes) and methanol oxidation (duplications of mxaF, xoxF1-5 genes), three pathways for one-carbon (C1) assimilation, and two methods of carbon storage (glycogen and polyhydroxyalkanoates). We also investigate the genetics of melanin production pathways as a key feature of the genus.

Interchangeability of class I and II fumarases in an obligate methanotroph Methylotuvimicrobium alcaliphilum 20Z.

The methanotrophic bacterium Methylotuvimicrobium alcaliphilum 20Z is an industrially promising candidate for bioconversion of methane into value-added chemicals. Here, we have study the metabolic consequences of the breaking in the tricarboxylic acid (TCA) cycle by fumarase knockout. Two fumarases belonging to non-homologous class I and II fumarases were obtained from the bacterium by heterologous expression in Escherichia coli. Class I fumarase (FumI) is a homodimeric enzyme catalyzing the reversible hydration of fumarate and mesaconate with activities of ~94 and ~81 U mg-1 protein, respectively. The enzyme exhibited high activity under aerobic conditions, which is a non-typical property for class I fumarases characterized to date. The calculation of kcat/S0.5 showed that the enzyme works effectively with either fumarate or mesaconate, but it is almost four times less specific to malate. Class II fumarase (FumC) has a tetrameric structure and equal activities of both fumarate hydration and malate dehydration (~45 U mg-1 protein). Using mutational analysis, it was shown that both forms of the enzyme are functionally interchangeable. The triple mutant strain 20Z-3E (ΔfumIΔfumCΔmae) deficient in the genes encoding the both fumarases and the malic enzyme accumulated 2.6 and 1.1 mmol g-1 DCW fumarate in the medium when growing on methane and methanol, respectively. Our data suggest the redundancy of the metabolic node in the TCA cycle making methanotroph attractive targets for modification, including generation of strains producing the valuable metabolites.

Methylomonas rapida sp. nov., a novel species of fast-growing, carotenoid-producing obligate methanotrophs with high biotechnological potential.

The genus Methylomonas accommodates strictly aerobic, obligate methanotrophs, with their sole carbon and energy sources restricted to methane and methanol. These bacteria inhabit oxic-anoxic interfaces of various freshwater habitats and have attracted considerable attention as potential producers of a single-cell protein. Here, we characterize two fast-growing representatives of this genus, strains 12 and MP1T, which are phylogenetically distinct from the currently described Methylomonas species (94.0-97.3 % 16S rRNA gene sequence similarity). Strains 12 and MP1T were isolated from freshwater sediments collected in Moscow and Krasnodar regions, respectively. Cells of these strains are Gram-negative, red-pigmented, highly motile thick rods that contain a type I intracytoplasmic membrane system and possess a particulate methane monooxygenase (pMMO) enzyme. These bacteria grow between 8 and 45 °C (optimum 35 °C) in a relatively narrow pH range of 5.5-7.3 (optimum pH 6.6-7.2). Major carotenoids synthesized by these methanotrophs are 4,4'-diaplycopene-4,4'-dioic acid, 1,1'-dihydroxy-3,4-didehydrolycopene and 4,4'-diaplycopenoic acid. High biomass yield, of up to 3.26 g CDW/l, is obtained during continuous cultivation of MP1T on natural gas in a bioreactor at a dilution rate of 0.22 h-1. The complete genome sequence of strain MP1T is 4.59 Mb in size; the DNA G + C content is 52.8 mol%. The genome encodes four rRNA operons, one pMMO operon and 4,216 proteins. The genome sequence displays 82-85 % average nucleotide identity to those of earlier described Methylomonas species. We propose to classify these bacteria as representing a novel species of the genus Methylomonas, M. rapida sp. nov., with the type strain MP1T (=KCTC 92586T = VKM B-3663T).

Genome sequence of the haloalkaliphilic methanotrophic bacterium Methylomicrobium alcaliphilum 20Z.

Methylomicrobium strains are widespread in saline environments. Here, we report the complete genome sequence of Methylomicrobium alcaliphilum 20Z, a haloalkaliphilic methanotrophic bacterium, which will provide the basis for detailed characterization of the core pathways of both single-carbon metabolism and responses to osmotic and high-pH stresses. Final assembly of the genome sequence revealed that this bacterium contains a 128-kb plasmid, making M. alcaliphilum 20Z the first methanotrophic bacterium of known genome sequence for which a plasmid has been reported.

Complete Genome Sequence of Methylococcus capsulatus MIR, a Methanotroph Capable of Growth on Methanol.

Methylococcus capsulatus MIR is an aerobic methanotroph that was isolated from an activated sludge sample and is capable of growth on methanol. The finished genome of strain MIR is 3.2 Mb in size. It encodes both MxaFI and XoxF methanol dehydrogenases, as well as three different isozymes of formate dehydrogenase.

Diversity and phylogeny of the ectoine biosynthesis genes in aerobic, moderately halophilic methylotrophic bacteria.

The genes of ectoine biosynthesis pathway were identified in six species of aerobic, slightly halophilic bacteria utilizing methane, methanol or methylamine. Two types of ectoine gene cluster organization were revealed in the methylotrophs. The gene cluster ectABC coding for diaminobutyric acid (DABA) acetyltransferase (EctA), DABA aminotransferase (EctB) and ectoine synthase (EctC) was found in methanotrophs Methylobacter marinus 7C and Methylomicrobium kenyense AMO1(T). In methanotroph Methylomicrobium alcaliphilum ML1, methanol-utilizers Methylophaga thalassica 33146(T) , Methylophaga alcalica M8 and methylamine-utilizer Methylarcula marina h1(T), the genes forming the ectABC-ask operon are preceded by ectR, encoding a putative transcriptional regulatory protein EctR. Phylogenetic relationships of the Ect proteins do not correlate with phylogenetic affiliation of the strains, thus implying that the ability of methylotrophs to produce ectoine is most likely the result of a horizontal transfer event.

Enzymes of an alternative pathway of glucose metabolism in obligate methanotrophs.

Aerobic methanotrophic bacteria utilize methane as a growth substrate but are unable to grow on any sugars. In this study we have shown that two obligate methanotrophs, Methylotuvimicrobium alcaliphilum 20Z and Methylobacter luteus IMV-B-3098, possess functional glucose dehydrogenase (GDH) and gluconate kinase (GntK). The recombinant GDHs from both methanotrophs were homotetrameric and strongly specific for glucose preferring NAD+ over NADP+. GDH from Mtm. alcaliphilum was most active at pH 10 (Vmax = 95 U/mg protein) and demonstrated very high Km for glucose (91.8 ± 3.8 mM). GDH from Mb. luteus was most active at pH 8.5 (Vmax = 43 U/mg protein) and had lower Km for glucose (16 ± 0.6 mM). The cells of two Mtm. alcaliphilum double mutants with deletions either of the genes encoding GDH and glucokinase (gdh─/glk─) or of the genes encoding gluconate kinase and glucokinase (gntk─/glk─) had the lower glycogen level and the higher contents of intracellular glucose and trehalose compared to the wild type strain. The gntk─/glk─ knockout mutant additionally accumulated gluconic acid. These data, along with bioinformatics analysis, demonstrate that glycogen derived free glucose can enter the Entner-Doudoroff pathway or the pentose phosphate cycle in methanotrophs, bypassing glycolysis via the gluconate shunt.

Expanding Characterized Diversity and the Pool of Complete Genome Sequences of Methylococcus Species, the Bacteria of High Environmental and Biotechnological Relevance.

The bacterial genus Methylococcus, which comprises aerobic thermotolerant methanotrophic cocci, was described half-a-century ago. Over the years, a member of this genus, Methylococcus capsulatus Bath, has become a major model organism to study genomic and metabolic basis of obligate methanotrophy. High biotechnological potential of fast-growing Methylococcus species, mainly as a promising source of feed protein, has also been recognized. Despite this big research attention, the currently cultured Methylococcus diversity is represented by members of the two species, M. capsulatus and M. geothermalis, while finished genome sequences are available only for two strains of these methanotrophs. This study extends the pool of phenotypically characterized Methylococcus strains with good-quality genome sequences by contributing four novel isolates of these bacteria from activated sludge, landfill cover soil, and freshwater sediments. The determined genome sizes of novel isolates varied between 3.2 and 4.0Mb. As revealed by the phylogenomic analysis, strains IO1, BH, and KN2 affiliate with M. capsulatus, while strain Mc7 may potentially represent a novel species. Highest temperature optima (45-50°C) and highest growth rates in bioreactor cultures (up to 0.3h-1) were recorded for strains obtained from activated sludge. The comparative analysis of all complete genomes of Methylococcus species revealed 4,485 gene clusters. Of these, pan-genome core comprised 2,331 genes (on average 51.9% of each genome), with the accessory genome containing 846 and 1,308 genes in the shell and the cloud, respectively. Independently of the isolation source, all strains of M. capsulatus displayed surprisingly high genome synteny and a striking similarity in gene content. Strain Mc7 from a landfill cover soil differed from other isolates by the high content of mobile genetic elements in the genome and a number of genome-encoded features missing in M. capsulatus, such as sucrose biosynthesis and the ability to scavenge phosphorus and sulfur from the environment.

Identification and characterization of EctR1, a new transcriptional regulator of the ectoine biosynthesis genes in the halotolerant methanotroph Methylomicrobium alcaliphilum 20Z.

Genes encoding key enzymes of the ectoine biosynthesis pathway in the halotolerant obligate methanotroph Methylomicrobium alcaliphilum 20Z have been shown to be organized into an ectABC-ask operon. Transcription of the ect operon is initiated from two promoters, ectAp(1) and ectAp(2) (ectAp(1)p(2)), similar to the sigma(70)-dependent promoters of Escherichia coli. Upstream of the gene cluster, an open reading frame (ectR1) encoding a MarR-like transcriptional regulator was identified. Investigation of the influence of EctR1 on the activity of the ectAp(1)p(2) promoters in wild-type M. alcaliphilum 20Z and ectR1 mutant strains suggested that EctR1 is a negative regulator of the ectABC-ask operon. Purified recombinant EctR1-His(6) specifically binds as a homodimer to the putative -10 motif of the ectAp(1) promoter. The EctR1 binding site contains a pseudopalindromic sequence (TATTTAGT-GT-ACTATATA) composed of 8-bp half-sites separated by 2 bp. Transcription of the ectR1 gene is initiated from a single sigma(70)-like promoter. The location of the EctR1 binding site between the transcriptional and translational start sites of the ectR1 gene suggests that EctR1 may regulate its own expression. The data presented suggest that in Methylomicrobium alcaliphilum 20Z, EctR1-mediated control of the transcription of the ect genes is not the single mechanism for the regulation of ectoine biosynthesis.

Ectoine degradation pathway in halotolerant methylotrophs.

BACKGROUND: Microorganisms living in saline environments are forced to regulate turgor via the synthesis of organic osmoprotective compounds. Microbial adaptation to fluctuations in external salinity includes degradation of compatible solutes. Here we have examined the biochemical pathway of degradation of the cyclic imino acid ectoine, the major osmoprotector in halotolerant methane-utilizing bacteria. METHODS: The BLAST search of the genes involved in ectoine degradation in the halotolerant methanotroph Methylotuvimicrobium alcaliphilum 20Z was performed with the reference sequences of Halomonas elongata. The genes for the key enzymes of the pathway were disrupted by insertion mutagenesis and the cellular metabolites in the methanol extracts of mutant cells were analyzed by HPLC. The doeA gene from Mm. alcaliphilum 20Z was heterologously expressed in Escherichia coli to identify the product of ectoine hydrolysis catalyzed by ectoine hydrolase DoeA. RESULTS: We have shown that the halotolerant methanotroph Mm. alcaliphilum 20Z possesses the doeBDAC gene cluster coding for putative ectoine hydrolase (DoeA), Nα-acetyl-L-2,4-diaminobutyrate deacetylase (DoeB), diaminobutyrate transaminase (DoeD) and aspartate-semialdehyde dehydrogenase (DoeC). The deletion of the doeA gene resulted in accumulation of the higher level of ectoine compared to the wild type strain. Nγ-acetyl-L-2,4-diaminobutyrate (Nγ-acetyl-DAB), a substrate for ectoine synthase, was found in the cytoplasm of the wild type strain. Nα-acetyl-L-2,4-diaminobutyrate (Nα-acetyl-DAB), a substrate for the DoeB enzyme, appeared in the cells as a result of exposure of the doeB mutant to low osmotic pressure. The genes for the enzymes involved in ectoine degradation were found in all aerobic methylotrophs capable of ectoine biosynthesis. These results provide the first evidence for the in vivo operation of the ectoine degradation pathway in methanotrophs and thus expand our understanding of the regulation mechanisms of bacterial osmoadaptation. CONCLUSIONS: During adaptation to the changes in external osmolarity, halophilic and halotolerant methylotrophs cleave ectoine, thereby entering the carbon and nitrogen of the compatible solute to the central metabolic pathways. The biochemical route of ectoine degradation in the halotolerant methanotroph Mm. alcaliphilum 20Z is similar to that in heterotrophic halophiles. We have shown that ectoine hydrolase DoeA in this methanotroph hydrolyzes ectoine with the formation of the only isomer: Nα-acetyl-DAB. All aerobic methylotrophs capable of ectoine biosynthesis harbor the genetic determinants for ectoine degradation.

Role of the malic enzyme in metabolism of the halotolerant methanotroph Methylotuvimicrobium alcaliphilum 20Z.

The bacteria utilizing methane as a growth substrate (methanotrophs) are important constituents of the biosphere. Methanotrophs mitigate the emission of anthropogenic and natural greenhouse gas methane to the environment and are the promising agents for future biotechnologies. Many aspects of CH4 bioconversion by methanotrophs require further clarification. This study was aimed at characterizing the biochemical properties of the malic enzyme (Mae) from the halotolerant obligate methanotroph Methylotuvimicrobium alcaliphilum 20Z. The His6-tagged Mae was obtained by heterologous expression in Escherichia coli BL21 (DE3) and purified by affinity metal chelating chromatography. As determined by gel filtration and non-denaturating gradient gel electrophoresis, the molecular mass of the native enzyme is 260 kDa. The homotetrameric Mae (65x4 kDa) catalyzed an irreversible NAD+-dependent reaction of L-malate decarboxylation into pyruvate with a specific activity of 32 ± 2 units mg-1 and Km value of 5.5 ± 0.8 mM for malate and 57 ± 5 µM for NAD+. The disruption of the mae gene by insertion mutagenesis resulted in a 20-fold increase in intracellular malate level in the mutant compared to the wild type strain. Based on both enzyme and mutant properties, we conclude that the malic enzyme is involved in the control of intracellular L-malate level in Mtm. alcaliphilum 20Z. Genomic analysis has revealed that Maes present in methanotrophs fall into two different clades in the amino acid-based phylogenetic tree, but no correlation of the division with taxonomic affiliations of the host bacteria was observed.

ATP- and Polyphosphate-Dependent Glucokinases from Aerobic Methanotrophs.

The genes encoding adenosine triphosphate (ATP)- and polyphosphate (polyP)-dependent glucokinases (Glk) were identified in the aerobic obligate methanotroph Methylomonas sp. 12. The recombinant proteins were obtained by the heterologous expression of the glk genes in Esherichia coli. ATP-Glk behaved as a multimeric protein consisting of di-, tri-, tetra-, penta- and hexamers with a subunit molecular mass of 35.5 kDa. ATP-Glk phosphorylated glucose and glucosamine using ATP (100% activity), uridine triphosphate (UTP) (85%) or guanosine triphosphate (GTP) (71%) as a phosphoryl donor and exhibited the highest activity in the presence of 5 mM Mg2+ at pH 7.5 and 65 °C but was fully inactivated after a short-term incubation at this temperature. According to a gel filtration in the presence of polyP, the polyP-dependent Glk was a dimeric protein (2 × 28 kDa). PolyP-Glk phosphorylated glucose, mannose, 2-deoxy-D-glucose, glucosamine and N-acetylglucosamine using polyP as the phosphoryl donor but not using nucleoside triphosphates. The Km values of ATP-Glk for glucose and ATP were about 78 µM, and the Km values of polyP-Glk for glucose and polyP(n=45) were 450 and 21 µM, respectively. The genomic analysis of methanotrophs showed that ATP-dependent glucokinase is present in all sequenced methanotrophs, with the exception of the genera Methylosinus and Methylocystis, whereas polyP-Glks were found in all species of the genus Methylomonas and in Methylomarinum vadi only. This work presents the first characterization of polyphosphate specific glucokinase in a methanotrophic bacterium.