Characterization of GlnK1 from Methanosarcina mazei strain Gö1: complementation of an Escherichia coli glnK mutant strain by GlnK1

Small protein mediates inhibition of ammonium transport in Methanosarcina mazei-an ancient mechanism?

IMPORTANCE

Small proteins containing fewer than 70 amino acids, which were previously disregarded due to computational prediction and biochemical detection challenges, have gained increased attention in the scientific community in recent years. However, the number of functionally characterized small proteins, especially in archaea, is still limited. Here, by using biochemical and genetic approaches, we demonstrate a crucial role of the small protein sP36 in the nitrogen metabolism of M. mazei, which modulates the ammonium transporter AmtB1 according to nitrogen availability. This modulation might represent an ancient archaeal mechanism of AmtB1 inhibition, in contrast to the well-studied uridylylation-dependent regulation in bacteria.

Immunogenic properties of the human gut-associated archaeon Methanomassiliicoccus luminyensis and its susceptibility to antimicrobial peptides

The methanogenic archaeon Methanomassiliicoccus luminyensis strain B10T was isolated from human feces just a few years ago. Due to its remarkable metabolic properties, particularly the degradation of trimethylamines, this strain was supposed to be used as "Archaebiotic" during metabolic disorders of the human intestine. However, there is still no data published regarding adaptations to the natural habitat of M. luminyensis as it has been shown for the other two reported mucosa-associated methanoarchaea. This study aimed at unraveling susceptibility of M. luminyensis to antimicrobial peptides as well as its immunogenicity. By using the established microtiter plate assay adapted to the anaerobic growth requirements of methanogenic archaea, we demonstrated that M. luminyensis is highly sensitive against LL32, a derivative of human cathelicidin (MIC = 2 μM). However, the strain was highly resistant against the porcine lysin NK-2 (MIC = 10 μM) and the synthetic antilipopolysaccharide peptide (Lpep) (MIC>10 μM) and overall differed from the two other methanoarchaea, Methanobrevibacter smithii and Methanosphaera stadtmanae in respect to AMP sensitivity. Moreover, only weak immunogenic potential of M. luminyensis was demonstrated using peripheral blood mononuclear cells (PBMCs) and monocyte-derived dendritic cells (moDCs) by determining release of pro-inflammatory cytokines. Overall, our findings clearly demonstrate that the archaeal gut inhabitant M. luminyensis is susceptible to the release of human-derived antimicrobial peptides and exhibits low immunogenicity towards human immune cells in vitro-revealing characteristics of a typical commensal gut microbe.

sRNA154 a newly identified regulator of nitrogen fixation in Methanosarcina mazei strain Gö1

Trans-encoded sRNA₁₅₄ is exclusively expressed under nitrogen (N)-deficiency in Methanosarcina mazei strain Gö1. The sRNA₁₅₄ deletion strain showed a significant decrease in growth under N-limitation, pointing toward a regulatory role of sRNA₁₅₄ in N-metabolism. Aiming to elucidate its regulatory function we characterized sRNA₁₅₄ by means of biochemical and genetic approaches. 24 homologs of sRNA₁₅₄ were identified in recently reported draft genomes of Methanosarcina strains, demonstrating high conservation in sequence and predicted secondary structure with two highly conserved single stranded loops. Transcriptome studies of sRNA₁₅₄ deletion mutants by an RNA-seq approach uncovered nifH- and nrpA-mRNA, encoding the α-subunit of nitrogenase and the transcriptional activator of the nitrogen fixation (nif)-operon, as potential targets besides other components of the N-metabolism. Furthermore, results obtained from stability, complementation and western blot analysis, as well as in silico target predictions combined with electrophoretic mobility shift-assays_, argue for a stabilizing effect of sRNA₁₅₄ on the polycistronic nif-mRNA and nrpA-mRNA by binding with both loops. Further identified N-related targets were studied, which demonstrates that translation initiation of glnA₂-mRNA, encoding glutamine synthetase2, appears to be affected by sRNA₁₅₄ masking the ribosome binding site, whereas glnA₁-mRNA appears to be stabilized by sRNA₁₅₄. Overall, we propose that sRNA₁₅₄ has a crucial regulatory role in N-metabolism in M. mazei by stabilizing the polycistronic mRNA encoding nitrogenase and glnA₁-mRNA, as well as allowing a feed forward regulation of nif-gene expression by stabilizing nrpA-mRNA. Consequently, sRNA₁₅₄ represents the first archaeal sRNA, for which a positive posttranscriptional regulation is demonstrated as well as inhibition of translation initiation.

pNEB193-derived suicide plasmids for gene deletion and protein expression in the methane-producing archaeon, Methanosarcina acetivorans

Gene deletion and protein expression are cornerstone procedures for studying metabolism in any organism, including methane-producing archaea (methanogens). Methanogens produce coenzymes and cofactors not found in most bacteria, therefore it is sometimes necessary to express and purify methanogen proteins from the natural host. Protein expression in the native organism is also useful when studying post-translational modifications and their effect on gene expression or enzyme activity. We have created several new suicide plasmids to complement existing genetic tools for use in the methanogen, Methanosarcina acetivorans. The new plasmids are derived from the commercially available Escherichia coli plasmid, pNEB193, and cannot replicate autonomously in methanogens. The designed plasmids facilitate markerless gene deletion, gene transcription, protein expression, and purification of proteins with cleavable affinity tags from the methanogen, M. acetivorans.

Biofilm formation of mucosa-associated methanoarchaeal strains

Although in nature most microorganisms are known to occur predominantly in consortia or biofilms, data on archaeal biofilm formation are in general scarce. Here, the ability of three methanoarchaeal strains, Methanobrevibacter smithii and Methanosphaera stadtmanae, which form part of the human gut microbiota, and the Methanosarcina mazei strain Gö1 to grow on different surfaces and form biofilms was investigated. All three strains adhered to the substrate mica and grew predominantly as bilayers on its surface as demonstrated by confocal laser scanning microscopy analyses, though the formation of multi-layered biofilms of Methanosphaera stadtmanae and Methanobrevibacter smithii was observed as well. Stable biofilm formation was further confirmed by scanning electron microscopy analysis. Methanosarcina mazei and Methanobrevibacter smithii also formed multi-layered biofilms in uncoated plastic μ-dishes^TM, which were very similar in morphology and reached a height of up to 40 μm. In contrast, biofilms formed by Methanosphaera stadtmanae reached only a height of 2 μm. Staining with the two lectins ConA and IB4 indicated that all three strains produced relatively low amounts of extracellular polysaccharides most likely containing glucose, mannose, and galactose. Taken together, this study provides the first evidence that methanoarchaea can develop and form biofilms on different substrates and thus, will contribute to our knowledge on the appearance and physiological role of Methanobrevibacter smithii and Methanosphaera stadtmanae in the human intestine.

Effects of antimicrobial peptides on methanogenic archaea

As members of the indigenous human microbiota found on several mucosal tissues, Methanobrevibacter smithii and Methanosphaera stadtmanae are exposed to the effects of antimicrobial peptides (AMPs) secreted by these epithelia. Although antimicrobial and molecular effects of AMPs on bacteria are well described, data for archaea are not available yet. Besides, it is not clear whether AMPs affect them as the archaeal cell envelope differs profoundly in terms of chemical composition and structure from that of bacteria. The effects of different synthetic AMPs on growth of M. smithii, M. stadtmanae, and Methanosarcina mazei were tested using a microtiter plate assay adapted to their anaerobic growth requirements. All three tested methanoarchaea were highly sensitive against derivatives of human cathelicidin, of porcine lysin, and a synthetic antilipopolysaccharide peptide (Lpep); however, sensitivities differed markedly among the methanoarchaeal strains. The potent AMP concentrations affecting growth were below 10 μM, whereas growth of Escherichia coli WBB01 was not affected at peptide concentrations up to 10 μM under the same anaerobic growth conditions. Atomic force microscopy and transmission electron microscopy revealed that the structural integrity of the methanoarchaeal cells is destroyed within 4 h after incubation with AMPs. The disruption of the cell envelope of M. smithii, M. stadtmanae, and M. mazei within a few minutes of exposure was verified by using LIVE/DEAD staining. Our results strongly suggest that the release of AMPs by eukaryotic epithelial cells is a potent defense mechanism targeting not only bacteria, but also methanoarchaea.

Establishing a markerless genetic exchange system for Methanosarcina mazei strain Gö1 for constructing chromosomal mutants of small RNA genes

A markerless genetic exchange system was successfully established in Methanosarcina mazei strain Gö1 using the hpt gene coding for hypoxanthine phosphoribosyltransferase. First, a chromosomal deletion mutant of the hpt gene was generated conferring resistance to the purine analog 8-aza-2,6-diaminopurine (8-ADP). The nonreplicating allelic exchange vector (pRS345) carrying the pac-resistance cassette for direct selection of chromosomal integration, and the hpt gene for counterselection was introduced into this strain. By a pop-in and ultimately pop-out event of the plasmid from the chromosome, allelic exchange is enabled. Using this system, we successfully generated a M. mazei deletion mutant of the gene encoding the regulatory non-coding RNA sRNA₁₅₄. Characterizing M. mazeiΔsRNA₁₅₄ under nitrogen limiting conditions demonstrated differential expression of at least three cytoplasmic proteins and reduced growth strongly arguing for a prominent role of sRNA₁₅₄ in regulation of nitrogen fixation by posttranscriptional regulation.

In vitro proof of direct regulation of glutamine synthetase by GlnK proteins in the extreme halophilic archaeon Haloferax mediterranei

Haloferax mediterranei is an extreme halophilic micro-organism belonging to the Archaea domain that was isolated from the Santa Pola solar salterns (Alicante, Spain) in 1983. The biochemistry of the proteins involved in nitrogen metabolism is being studied, but the knowledge of their regulation is very scarce at present. The PII superfamily is constituted by major regulators of nitrogen metabolism, which are widespread in prokaryotic and eukaryotic organisms. These trimeric proteins (12 kDa per subunit) have in Escherichia coli long been known to regulate GS (glutamine synthetase) activity via its adenylyltransferase/adenylyl-removing enzyme and, more recently, to be able to interact directly with this enzyme in methanogenic archaea. We have tested the possible role of PII proteins in the regulation of ammonium assimilation in our model organism and the results clearly indicate that the direct influence of GS by PII proteins can also take place in halophilic archaea, starting with the comprehension of nitrogen regulation in those organisms.

Formation of a cross-linking complex of dinitrogenase reductase-activating glycohydrolase (DRAG) with membrane proteins from Rhodospirillum rubrum chromatophores

Association of dinitrogenase reductase-activating glycohydrolase (DRAG) with membrane proteins of chromatophores has been investigated. The formation of a multicomponent complex between DRAG and membrane proteins was demonstrated in the presence of glutaraldehyde and EDC/NHS (N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride/hydroxy-2,5-dioxopyrrolidine-3-sulfonic acid sodium salt). Complex formation was observed both in native chromatophore membrane and in chromatophores treated with 0.5 M NaCl in the presence of homogeneous DRAG and glutaraldehyde in cross-reaction. The molecular weight of the complex was around 200 kD, which is consistent with the association of DRAG with three or more chromatophore membrane proteins. A specific complex with molecular weight of about 75 kD was formed only in the presence of EDC/NHS in the cross-linking reaction. It was demonstrated that ammonium transport protein and P11 protein are possible candidates for association with DRAG in chromatophore membranes.

Nitrogen Regulation in Bacteria and Archaea

A wide range of Bacteria and Archaea sense cellular 2-oxoglutarate (2OG) as an indicator of nitrogen limitation. 2OG sensor proteins are varied, but most of those studied belong to the PII superfamily. Within the PII superfamily, GlnB and GlnK represent a widespread family of homotrimeric proteins (GlnB-K) that bind and respond to 2OG and ATP. In some bacterial phyla, GlnB-K proteins are covalently modified, depending on enzymes that sense cellular glutamine as an indicator of nitrogen sufficiency. GlnB-K proteins are central clearing houses of nitrogen information and bind and modulate a variety of nitrogen assimilation regulators and enzymes. NifI(1) and NifI(2) comprise a second widespread family of PII proteins (NifI) that are heteromultimeric, respond to 2OG and ATP, and bind and regulate dinitrogenase in Euryarchaeota and many Bacteria.

Quantitative proteomic and microarray analysis of the archaeon Methanosarcina acetivorans grown with acetate versus methanol

Methanosarcina acetivorans strain C2A is an acetate- and methanol-utilizing methane-producing organism for which the genome, the largest yet sequenced among the Archaea, reveals extensive physiological diversity. LC linear ion trap-FTICR mass spectrometry was employed to analyze acetate- vs methanol-grown cells metabolically labeled with 14N vs 15N, respectively, to obtain quantitative protein abundance ratios. DNA microarray analyses of acetate- vs methanol-grown cells was also performed to determine gene expression ratios. The combined approaches were highly complementary, extending the physiological understanding of growth and methanogenesis. Of the 1081 proteins detected, 255 were > or =3-fold differentially abundant. DNA microarray analysis revealed 410 genes that were > or =2.5-fold differentially expressed of 1972 genes with detected expression. The ratios of differentially abundant proteins were in good agreement with expression ratios of the encoding genes. Taken together, the results suggest several novel roles for electron transport components specific to acetate-grown cells, including two flavodoxins each specific for growth on acetate or methanol. Protein abundance ratios indicated that duplicate CO dehydrogenase/acetyl-CoA complexes function in the conversion of acetate to methane. Surprisingly, the protein abundance and gene expression ratios indicated a general stress response in acetate- vs methanol-grown cells that included enzymes specific for polyphosphate accumulation and oxidative stress. The microarray analysis identified transcripts of several genes encoding regulatory proteins with identity to the PhoU, MarR, GlnK, and TetR families commonly found in the Bacteria domain. An analysis of neighboring genes suggested roles in controlling phosphate metabolism (PhoU), ammonia assimilation (GlnK), and molybdopterin cofactor biosynthesis (TetR). Finally, the proteomic and microarray results suggested roles for two-component regulatory systems specific for each growth substrate.

Protein-protein interactions in the archaeal transcriptional machinery: binding studies of isolated RNA polymerase subunits and transcription factors

Transcription in Archaea is directed by a pol II-like RNA polymerase and homologues of TBP and TFIIB (TFB) but the crystal structure of the archaeal enzyme and the subunits involved in recruitment of RNA polymerase to the promoter-TBP-TFB-complex are unknown. We described here the cloning expression and purification of 11 bacterially expressed subunits of the Pyrococcus furiosus RNAP. Protein interactions of subunits with each other and of archaeal transcription factors TFB and TFB with RNAP subunits were studied by Far-Western blotting and reconstitution of subcomplexes from single subunits in solution. In silico comparison of a consensus sequence of archaeal RNAP subunits with the sequence of yeast pol II subunits revealed a high degree of conservation of domains of the enzymes forming the cleft and catalytic center of the enzyme. Interaction studies with the large subunits were complicated by the low solubility of isolated subunits B, A', and A'', but an interaction network of the smaller subunits of the enzyme was established. Far-Western analyses identified subunit D as structurally important key polypeptide of RNAP involved in interactions with subunits B, L, N, and P and revealed also a strong interaction of subunits E' and F. Stable complexes consisting of subunits E' and F, of D and L and a BDLNP-subcomplex were reconstituted and purified. Gel shift analyses revealed an association of the BDLNP subcomplex with promoter-bound TBP-TFB. These results suggest a major role of subunit B (Rpb2) in RNAP recruitment to the TBP-TFB promoter complex.

Global transcriptional analysis of Methanosarcina mazei strain Gö1 under different nitrogen availabilities

Certain archaeal species can fix molecular nitrogen under nitrogen limiting conditions although little is known about this process at either the genetic or molecular level. To address this on a genome-wide scale, transcriptional analysis was performed on the model methanogen Methanosarcina mazei strain Gö1 using DNA-microarrays. The genomic expression patterns for cells grown under nitrogen fixing conditions versus nitrogen sufficiency (10 mM ammonium) revealed that approximately 5% of all genes are differentially expressed. Besides a small set of genes previously known to be up-regulated under nitrogen limitation, 14 additional genes involved in nitrogen metabolism were identified plus 10 genes encoding potential transcriptional regulators, 13 genes involved in carbon metabolism, 3 genes in general stress response, 8 putative transporter genes, and an additional 21 genes with unknown function. Quantitative reverse transcriptase PCR experiments confirmed the differential expression of a subset of these genes. Promoter analysis revealed a palindromic DNA motif centered nearby the transcriptional start point for several genes up-regulated under nitrogen limitation. A bioinformatics study demonstrated the presence of this motif in the up-stream region of 52 genes genome-wide, the majority of which showed nitrogen dependent differential transcription. We therefore hypothesize that this DNA element is involved in nitrogen control in M. mazei where it may act as a binding site for a regulatory protein.

Crystal structure of the archaeal ammonium transporter Amt-1 from Archaeoglobus fulgidus

Ammonium transporters (Amts) are integral membrane proteins found in all kingdoms of life that fulfill an essential function in the uptake of reduced nitrogen for biosynthetic purposes. Amt-1 is one of three Amts encoded in the genome of the hyperthermophilic archaeon Archaeoglobus fulgidus. The crystal structure of Amt-1 shows a compact trimer with 11 transmembrane helices per monomer and a central channel for substrate conduction in each monomer, similar to the known crystal structure of AmtB from Escherichia coli. Xenon derivatization has been used to identify apolar regions of Amt-1, emphasizing not only the hydrophobicity of the substrate channel but also the unexpected presence of extensive internal cavities that should be detrimental for protein stability. The substrates ammonium and methylammonium have been used for cocrystallization experiments with Amt-1, but the identification of binding sites that are distinct from water positions is not unambiguous. The well ordered cytoplasmic C terminus of the protein in the Amt-1 structure has allowed for the construction of a docking model between Amt-1 and a homology model for its physiological interaction partner, the P(II) protein GlnB-1. In this model, GlnB-1 binds tightly to the cytoplasmic face of the transporter, effectively blocking conduction through the three individual substrate channels.

Proteome of Methanosarcina acetivorans Part I: an expanded view of the biology of the cell

Methanosarcina acetivorans is representative of the genus that is distinguished from all other methane-producing genera by extensive metabolic diversity predicted from the large genome. In Part I of this study, two-dimensional gel electrophoresis and MALDI-TOF-TOF mass spectrometry was used to investigate the proteome of methanol- or acetate-grown M. acetivorans, with the goal of an initial characterization of the diversity of the proteins synthesized. A total of 412 proteins were identified, representing nearly 10% of the ORFs, with nearly 30% conserved hypothetical or hypothetical. Of the 412 proteins, 188 were found in both acetate- and methanol-grown cells, 122 were detected only in acetate-grown cells, and 102 only in methanol-grown cells. The results revealed the expression of a remarkable number of redundant genes which encode enzymes involved in the pathways for methanogenesis from methanol or acetate, suggesting an important role for the unusually high percentage of redundant genes in Methanosarcina species. Evidence was obtained for synthesis of a sodium-transporting oxidoreductase in acetate-grown cells, with the potential to function in energy conservation. Several transcriptional regulatory proteins were identified that also function in the Bacteria domain, raising questions regarding their interaction with the Archaea/Eucarya-type basal transcription apparatus. In addition, a significant number of proteins involved in protein folding were shown to be synthesized in methanol- and acetate-grown cells. These studies provide the first examination of the protein diversity of M. acetivorans.

Functional organization of a single nif cluster in the mesophilic archaeon Methanosarcina mazei strain Gö1

The mesophilic methanogenic archaeon Methanosarcina mazei strain Gö1 is able to utilize molecular nitrogen (N2) as its sole nitrogen source. We have identified and characterized a single nitrogen fixation (nif) gene cluster in M. mazei Gö1 with an approximate length of 9 kbp. Sequence analysis revealed seven genes with sequence similarities to nifH, nifI1, nifI2, nifD, nifK, nifE and nifN, similar to other diazotrophic methanogens and certain bacteria such as Clostridium acetobutylicum, with the two glnB-like genes (nifI1 and nifI2) located between nifH and nifD. Phylogenetic analysis of deduced amino acid sequences for the nitrogenase structural genes of M. mazei Gö1 showed that they are most closely related to Methanosarcina barkeri nif2 genes, and also closely resemble those for the corresponding nif products of the gram-positive bacterium C. acetobutylicum. Northern blot analysis and reverse transcription PCR analysis demonstrated that the M. mazei nif genes constitute an operon transcribed only under nitrogen starvation as a single 8 kb transcript. Sequence analysis revealed a palindromic sequence at the transcriptional start site in front of the M. mazei nifH gene, which may have a function in transcriptional regulation of the nif operon.

Development of genetic methods and construction of a chromosomal glnK1 mutant in Methanosarcina mazei strain Gö1

The methanogenic archaeon Methanosarcina mazei strain Gö1 has so far proven to be genetically intractable due to its low plating efficiency on solid medium and the lack of an effective transformation method. Here, we report the first significant improvement in plating efficiency (up to 10%), which was achieved by (1) selecting for a spontaneous mutant of M. mazei that shows significantly higher resistance to mechanical stress during spreading an agar plates, and (2) plating the cells in 0.5% top agar with trimethylamine as a carbon and energy source under a H2S-containing atmosphere (0.1%). Using this mutant we succeeded in establishing a liposome-mediated transformation protocol, which for the first time allowed genetic manipulation of the M. mazei Gö1 strain. We further report on the construction of the first chromosomal deletion mutant of M. mazei by means of homologous recombination. Characterization of this mutant strain revealed that M. mazei cells lacking a functional glnK1-gene exhibited a partial growth defect under nitrogen limitation when molecular nitrogen was used as the sole nitrogen source. Quantitative RT-PCR analysis, however, showed that genes involved in nitrogen assimilation or nitrogen fixation are transcribed in the glnK1 mutant as in the wild type. Thus, we propose that the archaeal GlnK1 protein is not directly involved in the transcriptional regulation of genes involved in nitrogen metabolism, but rather affects their protein products directly.

Unique mechanistic features of post-translational regulation of glutamine synthetase activity inMethanosarcina mazeistrain Gö1 in response to nitrogen availability

PII-like signal transduction proteins are found in all three domains of life and have been shown to play key roles in the control of bacterial nitrogen assimilation. This communication reports the first target protein of an archaeal PII-like protein, representing a novel PII receptor. The GlnK(1) protein of the methanogenic archaeon Methanosarcina mazei strain Go1 interacts and forms stable complexes with glutamine synthetase (GlnA(1)). Complex formation with GlnK(1) in the absence of metabolites inhibits the activity of GlnA(1). On the other hand, the activity of this enzyme is directly stimulated by the effector molecule 2-oxoglutarate. Moreover, 2-oxoglutarate antagonized the inhibitory effects of GlnK(1) on GlnA(1) activity but did not prevent GlnK(1)/GlnA(1) complex formation. On the basis of these findings, we hypothesize that besides the dominant effector molecule 2-oxoglutarate, the nitrogen sensor GlnK(1) allows finetuning control of the glutamine synthetase activity under changing nitrogen availabilities and propose the following model. (i) Under nitrogen limitation, increasing concentrations of 2-oxoglutarate stimulate maximal GlnA(1) activity and transform GlnA(1) into an activated conformation, which prevents inhibition by GlnK(1). (ii) Upon a shift to nitrogen sufficiency after a period of nitrogen limitation, GlnA(1) activity is reduced by decreasing internal 2-oxoglutarate concentrations through diminished direct activation and by GlnK(1) inhibition.

GlnK effects complex formation between NifA and NifL in Klebsiella pneumoniae

In Klebsiella pneumoniae, the nif specific transcriptional activator NifA is inhibited by NifL in response to molecular oxygen and ammonium. Here, we demonstrate complex formation between NifL and NifA (approximately 1 : 1 ratio), when synthesized in the presence of oxygen and/or ammonium. Under simultaneous oxygen- and nitrogen-limitation, significant but fewer NifL-NifA complexes (approximately 1%) were formed in the cytoplasm as a majority of NifL was sequestered to the cytoplasmic membrane. These findings indicate that inhibition of NifA in the presence of oxygen and/or ammonium occurs via direct NifL interaction and formation of those inhibitory NifL-NifA complexes appears to be directly and exclusively dependent on the localization of NifL in the cytoplasm. We further observed evidence that the nitrogen sensory protein GlnK forms a trimeric complex with NifL and NifA under nitrogen limitation. Binding of GlnK to NifL-NifA was specific; however the amount of GlnK within these complexes was small. Finally, two lines of evidence were obtained that under anaerobic conditions but in the presence of ammonium additional NtrC-independent GlnK synthesis inhibited the formation of stable inhibitory NifL-NifA complexes. Thus, we propose that the NifL-NifA-GlnK complex reflects a transitional structure and hypothesize that under nitrogen-limitation, GlnK interacts with the inhibitory NifL-NifA complex, resulting in its dissociation.