Quorum Sensing in Methanosaeta harundinacea 6Ac: Difference between revisions

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=Overview=
=Overview=


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=Quorum Sensing=
=Quorum Sensing=


Quorum sensing is a cell density-dependent process of gene regulation by which unicellular organisms can communicate with others of the same species and co-ordinate their behaviours (Miller and Bassler 2001). Signal molecules called autoinducers are constitutively expressed and secreted out of the cell, which then get taken up by members of the other species present in the area. As such, autoinducer concentration in the immediate environment increases as cell density increases and given that enough autoinducers are uptaken, a threshold concentration is reached inside the cells which triggers a change in gene expression. The cells can then co-ordinate a number of processes together, such as biofilm formation, sporulation (Miller and Bassler 2001, production of virulence factors such as in ''Pseudomonas aeruginosa'' (Pesci et al. 1997) , or in the case of ''Vibrio fischeri'', bioluminescence (Hastings and Greenberg 1999). In Gram-positive bacteria, oligopeptides serve as autoinducers, and Gram-negative bacteria use acyl homoserine lactones (AHLs) (Miller and Bassler 2001).
Quorum sensing is a cell density-dependent process of gene regulation by which unicellular organisms can communicate with others of the same species and co-ordinate their behaviours (Miller and Bassler 2001). Signal molecules called autoinducers are constitutively expressed and secreted out of the cell, which then get taken up by members of the other species present in the area. As such, autoinducer concentration in the immediate environment increases as cell density increases and given that enough autoinducers are uptaken, a threshold concentration is reached inside the cells which triggers a change in gene expression. The cells can then co-ordinate a number of processes together, such as biofilm formation, sporulation (Miller and Bassler 2001, production of virulence factors such as in ''Pseudomonas aeruginosa'' (Pesci et al. 1997) , or in the case of ''Vibrio fischeri'', bioluminescence (Hastings and Greenberg 1999). In Gram-positive bacteria, oligopeptides serve as autoinducers, and Gram-negative bacteria use acyl homoserine lactones (AHLs) (Miller and Bassler 2001).


More on [http://www.annualreviews.org.ezproxy.library.ubc.ca/doi/pdf/10.1146/annurev.cellbio.21.012704.131001 quorum sensing]
More on [http://www.annualreviews.org.ezproxy.library.ubc.ca/doi/pdf/10.1146/annurev.cellbio.21.012704.131001 quorum sensing]
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[http://stke.sciencemag.org.ezproxy.library.ubc.ca/cgi/reprint/pnas;99/5/3129.pdf Quorum sensing related virulence in ''Vibrio cholerae'']
[http://stke.sciencemag.org.ezproxy.library.ubc.ca/cgi/reprint/pnas;99/5/3129.pdf Quorum sensing related virulence in ''Vibrio cholerae'']


[[File:Autoinducers.jpeg|400px|thumb|right| Left: the 3 different carboxyl-AHLs used by M. harundinacea 6Ac. Right: acyl homoserine lactone (AHL), the autoinducer used by Gram negative bacteria. C4-HSL is shown.]]
[[File:Autoinducers.jpeg|400px|thumb|right| Left: the 3 different carboxyl-AHLs used by M. harundinacea 6Ac (Zhang et al. 2012). Right: acyl homoserine lactone (AHL), the autoinducer used by Gram negative bacteria. C4-HSL is shown (Davis et al. 2010).]]


==Autoinducers in ''M. harundinacea'' 6Ac==
==Autoinducers in ''M. harundinacea'' 6Ac==


The autoinducers used by ''M. harundinacea'' 6Ac are carboxyl-AHLs, which are essentially AHLs with a carboxyl moiety attached to the nitrogen (Zhang et al. 2012). There are 3 slight variants synthesized (see figure 1), which are N-carboxyl-C14-HSL (homoserine lactone), N-carboxyl-C12-HSL, and N-carboxyl-C10-HSL (Zhang et al. 2012). These are made by the enzyme AHL synthase, encoded for by the ''fliI'' gene, which is basally transcribed(Zhang et al. 2012). The sensor protein for the carboxyl-AHLs, FilR, is coded for by the ''filR'' gene which is also basally transcribed, and the protein binds the autoinducers when they reach a threshold concentration (Zhang et al. 2012), the concentration which is an indication of increased cell-density in the area. The FilR-autoinducer complex binds to certain genes in the DNA resulting in up-regulation of their transcription, and down-regulation of other genes, which results in changes in physiology of the cells as well as a different metabolic pattern (Zhang et al. 2012).
The autoinducers used by ''M. harundinacea'' 6Ac are carboxyl-AHLs, which are essentially AHLs with a carboxyl moiety attached to the nitrogen (Zhang et al. 2012). There are 3 slight variants synthesized (see figure 1), which are N-carboxyl-C14-HSL (homoserine lactone), N-carboxyl-C12-HSL, and N-carboxyl-C10-HSL (Zhang et al. 2012). These are made by the enzyme AHL synthase, encoded for by the ''fliI'' gene, which is basally transcribed(Zhang et al. 2012). The sensor protein for the carboxyl-AHLs, FilR, is coded for by the ''filR'' gene which is also basally transcribed, and the protein binds the autoinducers when they reach a threshold concentration (Zhang et al. 2012), the concentration which is an indication of increased cell-density in the area. The FilR-autoinducer complex binds to certain genes in the DNA resulting in up-regulation of their transcription, and down-regulation of other genes, which results in changes in physiology of the cells as well as a different metabolic pattern (Zhang et al. 2012).
[[File:EM spacer plug.jpeg|300px|thumb|left| Negatively stained EM micrograph depicting a spacer plug which has been separated from the filament.  The plug is firmly attached to the sheath, and is composed of numerous concentric rings.]]
[[File:EM spacer plug.jpeg|300px|thumb|left| Negatively stained EM micrograph depicting a spacer plug which has been separated from the filament.  The plug is firmly attached to the sheath, and is composed of numerous concentric rings (Beveridge et al. 1986).]]




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When the cell density is low, ''M. harundinacea'' 6Ac exist as short, singular rod-shaped cells (Ma et al. 2006), but when the cell density is high the archaeon’s physiology drastically changes. Chains of cells are stacked end to end forming filaments that can be greater than 200 microns, in contrast to a length of 3-5 microns as single cells (Guo et al. 2009). Beveridge et al. (1986) found that the filaments consist of four major parts: the cells lined up in a chain, spacer plugs which separated each individual cell as well as plugging the two ends of the filament, an amorphous granular matrix, and encasing everything a proteinaceous sheath. The spacer plugs are anchored to the sheath, and appear to have multiple layers. The granular matrix is enclosed by the sheath and the plugs and completely encloses individual cells. The protein sheath is striated and composed of a highly crystalline structure. This lends it tremendous strength against pressure, as well as other physical and chemical disruption (Beveridge et al. 1986). As such, when ''M. harundinacea'' 6Ac form into filaments, they can better withstand unfavourable environment conditions and thus have an edge over other micro-organisms in the area.  Note that Beveridge et al. (1986) described filaments formed by ''Methanothrix concilii'', which have been renamed to ''Methanosaeta concilii'' as elucidated by Patel and Sprott (1990). Given that there is a 92.5% similarity between ''M. concilii'' and ''M. harundinacea'' 6Ac based on 16S rRNA sequences (Ma et al. 2006), the filament descriptions are likely accurate for ''M. harundinacea'' 6Ac as well.
When the cell density is low, ''M. harundinacea'' 6Ac exist as short, singular rod-shaped cells (Ma et al. 2006), but when the cell density is high the archaeon’s physiology drastically changes. Chains of cells are stacked end to end forming filaments that can be greater than 200 microns, in contrast to a length of 3-5 microns as single cells (Guo et al. 2009). Beveridge et al. (1986) found that the filaments consist of four major parts: the cells lined up in a chain, spacer plugs which separated each individual cell as well as plugging the two ends of the filament, an amorphous granular matrix, and encasing everything a proteinaceous sheath. The spacer plugs are anchored to the sheath, and appear to have multiple layers. The granular matrix is enclosed by the sheath and the plugs and completely encloses individual cells. The protein sheath is striated and composed of a highly crystalline structure. This lends it tremendous strength against pressure, as well as other physical and chemical disruption (Beveridge et al. 1986). As such, when ''M. harundinacea'' 6Ac form into filaments, they can better withstand unfavourable environment conditions and thus have an edge over other micro-organisms in the area.  Note that Beveridge et al. (1986) described filaments formed by ''Methanothrix concilii'', which have been renamed to ''Methanosaeta concilii'' as elucidated by Patel and Sprott (1990). Given that there is a 92.5% similarity between ''M. concilii'' and ''M. harundinacea'' 6Ac based on 16S rRNA sequences (Ma et al. 2006), the filament descriptions are likely accurate for ''M. harundinacea'' 6Ac as well.


[[File:Filament segment.jpeg|600px|thumb|right| A segment of the filament (3 individual cells) formed by M. harundinacea. The four major components of the filaments, the outer protein sheath, the spacer plugs, granular matrix, and the individual archaea are shown.]]
[[File:Filament segment.jpeg|600px|thumb|right| A segment of the filament (3 individual cells) formed by M. harundinacea. The four major components of the filaments, the outer protein sheath, the spacer plugs, granular matrix, and the individual archaea are shown (Beveridge et al. 1986).]]
 
 




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The metabolic patterns between the 2 distinct morphologies are significantly different. Both methane production and the reaction rate of methanogenesis are higher in the filamentous cells than in the short rods (Zhang et al. 2012). It was also shown by Zhang et al. (2012) that the filamentous cells produce lower levels of cellular proteins compared to the short cells, and had less biomass. The significance of this is that ''M. harundinacea'' 6Ac can better compete with other micro-organisms for resources in the environment, such as for acetate with acetoclastic microbes, and as well they can divide quicker due to faster metabolism and the lower biomass required, and outgrow any competitors.
The metabolic patterns between the 2 distinct morphologies are significantly different. Both methane production and the reaction rate of methanogenesis are higher in the filamentous cells than in the short rods (Zhang et al. 2012). It was also shown by Zhang et al. (2012) that the filamentous cells produce lower levels of cellular proteins compared to the short cells, and had less biomass. The significance of this is that ''M. harundinacea'' 6Ac can better compete with other micro-organisms for resources in the environment, such as for acetate with acetoclastic microbes, and as well they can divide quicker due to faster metabolism and the lower biomass required, and outgrow any competitors.


=Metabolism=
=Similarities to Bacterial Quorum Sensing=


''Ignicoccus'' species are [http://en.wikipedia.org/wiki/Chemolithoautotroph chemolithoautotrophs] that use molecular hydrogen as the inorganic electron donor and elemental sulphur as the inorganic terminal electron acceptor[[#References|[1]]] . The reduction of the elemental sulphur results in the production of hydrogen sulphide gas.  
There are quite a few similarities between the quorum sensing mechanism of ''M. harundinacea'' 6Ac and that of bacteria. The ''filI-filR'' system is a homolog of a universal quorum sensing system in bacteria, the [http://www-ncbi-nlm-nih-gov.ezproxy.library.ubc.ca/pmc/articles/PMC205046/pdf/jbacter00020-0013.pdf ''luxI-luxR'' system], which essentially works the same way. The ''luxI'' gene encodes a protein which synthesizes the autoinducer, and the autoinducer is bound by the sensor protein encoded by ''luxR'', and the LuxR-autoinducer complex facilitates the changes in gene expression (Miller and Bassler 2001).
FilI synthesizes the autoinducer carboxyl-AHL, which as mentioned above is only one carboxyl moiety away from being an AHL, which is the autoinducer utilized by Gram-negative bacteria. This suggests that possible horizontal gene transfer, the flow of genetic information between prokaryotes of different species (Syvanen M. 2012), might have occurred between bacterial species and the archaeon. In addition, several other ''Methanosaeta'' spp.(such as [http://ijs.sgmjournals.org.ezproxy.library.ubc.ca/content/42/3/463.full.pdf+html ''Methanosaeta thermophila''], and other methanogenic archaea (such as [http://www-ncbi-nlm-nih-gov.ezproxy.library.ubc.ca/pmc/articles/PMC203386/ ''Methanosarcina mazei''] have been found to respond to carboxyl-AHLs and undergo morphological changes in a cell density-dependent manner (Zhang et al. 2012), and so AHL-based quorum sensing may be a universal mechanism in prokaroytes.


''Ignicoccus'' are autotrophs in that they fix their own carbon dioxide into organic molecules. The carbon dioxide fixation process they use is a novel process called [http://www.pnas.org/content/105/22/7851.full a dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle] that involves 14 different enzymes[[#References|[8]]] .
=Summary=


Members of the ''Ignicoccus'' genus are able to use ammonium as a nitrogen source.
Quorum sensing is a demonstrated process in the archaeon ''M. harundinacea'' 6Ac, via the usage of carboxyl-AHLs and the FilR protein, and the changes in gene expression result in the formation of filaments composed of 4 major parts, the cells, spacer plugs, extracellular matrix, and a sheath. As well the filamentous cells have a higher and faster methane output rate, and have decreased protein production, and as a result a lower biomass. These changes enable the archaea to work together as a group and give them an edge over other microbes in the environment, namely being more resistant to unfavourable environment conditions, and being able replicate faster.


==Growth Conditions==
=References=
 
Because members of the ''Ignicoccus''  genus are [http://en.wikipedia.org/wiki/Hyperthermophile hyperthermophiles] and obligate anaerobes, it is not surprising that their growth conditions are very complex. They are grown in a liquid medium known as ½ SME ''Ignicoccus''  which is a solution of synthetic sea water which is then made anaerobic.
 
Grown in this media at their optimal growth temperature of 90C, the members of the ''Ignicoccus''  genus typically reach a cell density of ~4x107cells/mL[[#References|[1]]] .
 
The addition of [http://en.wikipedia.org/wiki/Yeast_extract yeast extract] to the ½ SME media has been shown to stimulate the growth and increase maximum cell density achieved. The mechanism by which this is achieved is not known[[#References|[1]]] .


Beveridge T.J., Patel G.B., Harris B.J. and Dennis Sprott G. “The Ultrastructure of ''Methanothrix concilii'', a Mesophilic Aceticlastic Methanogen.” Canadian Journal of Microbiology, 1986, DOI: 10.1139/m86-128


=Symbiosis=
Davis B.M., Jensen R., Williams P., and O’Shea P. “The Interaction of N-Acylhomoserine Lactone Quorum Sensing Signaling Molecules with Biological Membranes: Implications for Inter-Kingdom Signaling.” PLoS ONE, 2010, 5(10):e13522.doi:10.1371/journal.pone.0013522
 
''Ignicoccus hospitalis''  is the only member of the genus ''Ignicoccus'' that has been shown to have an extensive [http://en.wikipedia.org/wiki/Symbiosis symbiotic relationship] with another organism.
 
''Ignicoccus hospitalis''  has been shown to engage in symbiosis with ''Nanoarchaeum equitans'' . ''Nanoarchaeum equitans''  is a very small coccoid species with a cell diameter of 0.4 µm[[#References|[9]]] . Genome analysis has provided much of the known information about this species.
 
To further complicate the symbiotic relationship between both species, it’s been observed that the presence of ''Nanoarchaeum equitans''  on the surface of ''Ignicoccus hospitalis''  somehow inhibits the cell replication of ''Ignicoccus hospitalis'' . How or why this occurs has not yet been elucidated[[#References|[3]]] .
 
[[File:Urzwerg.jpg|300px|thumb|right| ''Ignicoccus hospitalis'' with two attached  ''Nanoarchaeum equitans'' cells.]]
 
[[File:IhNeRelationship2 jpeg.jpg|250px|thumb|left| Epifluoroscence micrographs of an ''Ignicoccus hospitalis''and ''Nanoarchaeum equitans'' coculture stained with BacLight at various time points. Living cells stain green while dead cells stain red. (A) Exponential growth phase 3.25 hours after inoculation. (B) Transition into the stationary phase 7.5 hours after inoculation. (C) Stationary phase 10 hours after inoculation. (D) Stationary phase 23 hours after inoculation.]]
 
 
==''Nanoarchaeum equitans''==
 
''Nanoarchaeum equitans'' has the smallest non-viral genome ever sequenced at 491kb[[#References|[9]]] . Analysis of the genome sequence indicates that 95% of the predicted proteins and stable RNA molecules are somehow involved in repair and replication of the cell and its genome[[#References|[3]]] .
 
Analysis of the genome also showed that ''Nanoarchaeum equitans''  lacks nearly all genes known to be required in amino acid, nucleotide, cofactor and lipid metabolism. This is partially supported by the evidence that ''Nanoarchaeum equitans''  has been shown to derive its cell membrane from its host ''Ignicoccus hospitalis''  cell membrane. The direct contact observed between ''Nanoarchaeum equitans'' and ''Ignicoccus hospitalis''  is hypothesized to form a pore between the two organisms in order to exchange metabolites or substrates (likely from ''Ignicoccus hospitalis''  towards ''Nanoarchaeum equitans'' due to the parasitic relationship). The exchange of periplasmic vesicles is not thought to be involved in metabolite or substrate exchange despite the presence of vesicles in the periplasm of ''Ignicoccus hospitalis'' .
 
These analyses of the ''Nanoarchaeum equitans'' genome support the fact of the extensive symbiotic relationship between ''Nanoarchaeum equitans'' and ''Ignicoccus hospitalis''. However, it has not yet been proven that it is a strictly parasitic relationship and further research may prove that there is a commensal relationship between the two species.
 
=References=
 
(1) Burggraf S., Huber H., Mayer T., Rachel R., Stetter K.O. and Wyschkony I. ” Ignicoccus gen. nov., a novel genus of hyperthermophilic, chemolithoautotrophic Archaea, represented by two new species, Ignicoccus islandicus sp. nov. and Ignicoccus pacificus sp. nov.” International Journal of Systematic and Evolutionary Microbiology, 2000, Volume 50.


(2) Naether D.J. and Rachel R. “The outer membrane of the hyperthermophilic archaeon Ignicoccus: dynamics, ultrastructure and composition.” Biochemical Society Transactions, 2004, Volume 32, part 2.
Ferry, J.G. “Acetate-Based Methane Production.” Bioenergy, 2008, pp. 155-170


(3) Giannone R.J., Heimerl T., Hettich R.L., Huber H., Karpinets T., Keller M., Kueper U., Podar M. and Rachel R. “Proteomic Characterization of Cellular and Molecular Processes that Enable the Nanoarchaeum equitans- Ignicoccus hospitalis Relationship.” PLoS ONE, 2011, Volume 6, Issue 8.
Guo X., Zhang G., Liu X. and Dong X. “Detection of the Quorum Sensing Signals in Methanogenic Archaea.” Acta Microbiologica Sinica, 2011, 51(9):1200-1204


(4) Eisenreich W., Gallenberger M., Huber H., Jahn U., Junglas B., Paper W., Rachel R. and Stetter K.O. “Nanoarchaeum equitans and Ignicoccus hospitalis: New Insights into a Unique, Intimate Association of Two Archaea.Journal of Bacteriology, 2008, DOI: 10.1128/JB.01731-07.
Hastings J.W. and Greenberg E.P. “Quorum Sensing: the Explanation of a Curious Phenomenon Reveals a Common Characteristic of Bacteria”. Journal of Bacteriology, 1999, 181(9):2667-2668


(5) Grosjean E., Huber H., Jahn U., Sturt H, and Summons R. “Composition of the lipids of Nanoarchaeum equitans and their origin from its host Ignicoccus sp. strain KIN4/I.” Arch Microbiol, 2004, DOI: 10.1007/s00203-004-0725-x.
Ma K., Liu X., and Dong X. “''Methanosaeta harundinacea'' sp nov., a novel acetate-scavenging methanogen isolated from a UASB reactor.” International Journal of Systematic and Evolutionary Microbiology, 2006, DOI: 10.1099/ijs.0.63887-0


(6) Briegel A., Burghardt T., Huber H., Junglas B., Rachel R., Walther P. and Wirth R. “Ignicoccus hospitalis and Nanoarchaeum equitans: ultrastructure, cell–cell interaction, and 3D reconstruction from serial sections of freeze-substituted cells and by electron cryotomography.”  Arch Microbiol, 2008, DOI 10.1007/s00203-008-0402-6.
Miller M.B., and Bassler B.L. “Quorum Sensing in Bacteria.” Annual Review of Microbiology, 2001, DOI: 10.1146/annurev.micro.55.1.165


(7) Burghardt T., Huber H., Junglas B., Naether D.J. and Rachel R. “The dominating outer membrane protein of the hyperthermophilic Archaeum Ignicoccus hospitalis: a novel pore-forming complex.” Molecular Microbiology, 2007, Volume 63.
Patel, B.G. and Dennis Sprott G. “''Methanosaeta concilii'' gen. nov., sp. nov. (“''Methanothrix concilii''”) and ''Methanosaeta thermoacetophila'' nom. rev., comb. nov.” International Journal of Systematic and Evolutionary Microbiology, 1990, DOI: 10.1099/00207713-40-1-79


(8) Berg I.A., Eisenreich W., Eylert E., Fuchs G., Gallenberger M., Huber H.,Jahn U. and Kockelkorn D. “A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis.” PNAS, 2008, Volume 105, issue 22.
Pesci E.C., Pearson J.P., Seed P.C., and Iglewski B.H. “Regulation of las and rhl quorum sensing in ''Pseudomonas aeruginosa''”. Journal of Bacteriology, 1997, 179(10):3127-3132


(9) Brochier C., Gribaldo S.,  Zivanovic Y., Confalonieri F. and Forterre P. “Nanoarchaea: representatives of a novel archaeal phylum or a fast-evolving euryarchaeal lineage related to Thermococcales?” Genome Biology 2005, DOI:10.1186/gb-2005-6-5-r42.
Syvanen M. “Evolutionary Implications of Horizontal Gene Transfer”. Annual Review of Genetics, 2012, DOI: 10.1146/annurev-genet-110711-155529


(10) Huber H., Rachel R., Riehl S. and Wyschkony I. “The ultrastructure of Ignicoccus: Evidence for a novel outer membrane and for intracellular vesicle budding in an archaeon.” Archaea, 2002, Volume 1.
Zhang G., Zhang F., Ding G., Li J., Guo X., Zhu J., Zhou L., Cai S., Liu X., Luo Y., Zhang G., Shi W. and Dong X. “Acyl Homoserine Lactone-Based Quorum Sensing in a Methanogenic Archaeon.” The ISME Journal, 2012, DOI:10.1038/ismej.2011.203s

Latest revision as of 15:21, 2 October 2015

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Overview

The archaeon Methanosaeta harundinacea 6Ac is part of a family of aceticlastic methanogens who exclusively catabolize acetate (Ferry 2008). It is rod-shaped, non-motile, and usually found singly or in pairs, but when grown in acetate with high cell density, the cells form into filaments which are self-contained (Ma et. al 2006), products of quorum sensing.

Quorum Sensing

Quorum sensing is a cell density-dependent process of gene regulation by which unicellular organisms can communicate with others of the same species and co-ordinate their behaviours (Miller and Bassler 2001). Signal molecules called autoinducers are constitutively expressed and secreted out of the cell, which then get taken up by members of the other species present in the area. As such, autoinducer concentration in the immediate environment increases as cell density increases and given that enough autoinducers are uptaken, a threshold concentration is reached inside the cells which triggers a change in gene expression. The cells can then co-ordinate a number of processes together, such as biofilm formation, sporulation (Miller and Bassler 2001, production of virulence factors such as in Pseudomonas aeruginosa (Pesci et al. 1997) , or in the case of Vibrio fischeri, bioluminescence (Hastings and Greenberg 1999). In Gram-positive bacteria, oligopeptides serve as autoinducers, and Gram-negative bacteria use acyl homoserine lactones (AHLs) (Miller and Bassler 2001).

More on quorum sensing

Quorum sensing related virulence in Vibrio cholerae

Left: the 3 different carboxyl-AHLs used by M. harundinacea 6Ac (Zhang et al. 2012). Right: acyl homoserine lactone (AHL), the autoinducer used by Gram negative bacteria. C4-HSL is shown (Davis et al. 2010).

Autoinducers in M. harundinacea 6Ac

The autoinducers used by M. harundinacea 6Ac are carboxyl-AHLs, which are essentially AHLs with a carboxyl moiety attached to the nitrogen (Zhang et al. 2012). There are 3 slight variants synthesized (see figure 1), which are N-carboxyl-C14-HSL (homoserine lactone), N-carboxyl-C12-HSL, and N-carboxyl-C10-HSL (Zhang et al. 2012). These are made by the enzyme AHL synthase, encoded for by the fliI gene, which is basally transcribed(Zhang et al. 2012). The sensor protein for the carboxyl-AHLs, FilR, is coded for by the filR gene which is also basally transcribed, and the protein binds the autoinducers when they reach a threshold concentration (Zhang et al. 2012), the concentration which is an indication of increased cell-density in the area. The FilR-autoinducer complex binds to certain genes in the DNA resulting in up-regulation of their transcription, and down-regulation of other genes, which results in changes in physiology of the cells as well as a different metabolic pattern (Zhang et al. 2012).

Negatively stained EM micrograph depicting a spacer plug which has been separated from the filament. The plug is firmly attached to the sheath, and is composed of numerous concentric rings (Beveridge et al. 1986).


Changes in Physiology

When the cell density is low, M. harundinacea 6Ac exist as short, singular rod-shaped cells (Ma et al. 2006), but when the cell density is high the archaeon’s physiology drastically changes. Chains of cells are stacked end to end forming filaments that can be greater than 200 microns, in contrast to a length of 3-5 microns as single cells (Guo et al. 2009). Beveridge et al. (1986) found that the filaments consist of four major parts: the cells lined up in a chain, spacer plugs which separated each individual cell as well as plugging the two ends of the filament, an amorphous granular matrix, and encasing everything a proteinaceous sheath. The spacer plugs are anchored to the sheath, and appear to have multiple layers. The granular matrix is enclosed by the sheath and the plugs and completely encloses individual cells. The protein sheath is striated and composed of a highly crystalline structure. This lends it tremendous strength against pressure, as well as other physical and chemical disruption (Beveridge et al. 1986). As such, when M. harundinacea 6Ac form into filaments, they can better withstand unfavourable environment conditions and thus have an edge over other micro-organisms in the area. Note that Beveridge et al. (1986) described filaments formed by Methanothrix concilii, which have been renamed to Methanosaeta concilii as elucidated by Patel and Sprott (1990). Given that there is a 92.5% similarity between M. concilii and M. harundinacea 6Ac based on 16S rRNA sequences (Ma et al. 2006), the filament descriptions are likely accurate for M. harundinacea 6Ac as well.

A segment of the filament (3 individual cells) formed by M. harundinacea. The four major components of the filaments, the outer protein sheath, the spacer plugs, granular matrix, and the individual archaea are shown (Beveridge et al. 1986).






Changes in Metabolism

The metabolic patterns between the 2 distinct morphologies are significantly different. Both methane production and the reaction rate of methanogenesis are higher in the filamentous cells than in the short rods (Zhang et al. 2012). It was also shown by Zhang et al. (2012) that the filamentous cells produce lower levels of cellular proteins compared to the short cells, and had less biomass. The significance of this is that M. harundinacea 6Ac can better compete with other micro-organisms for resources in the environment, such as for acetate with acetoclastic microbes, and as well they can divide quicker due to faster metabolism and the lower biomass required, and outgrow any competitors.

Similarities to Bacterial Quorum Sensing

There are quite a few similarities between the quorum sensing mechanism of M. harundinacea 6Ac and that of bacteria. The filI-filR system is a homolog of a universal quorum sensing system in bacteria, the luxI-luxR system, which essentially works the same way. The luxI gene encodes a protein which synthesizes the autoinducer, and the autoinducer is bound by the sensor protein encoded by luxR, and the LuxR-autoinducer complex facilitates the changes in gene expression (Miller and Bassler 2001). FilI synthesizes the autoinducer carboxyl-AHL, which as mentioned above is only one carboxyl moiety away from being an AHL, which is the autoinducer utilized by Gram-negative bacteria. This suggests that possible horizontal gene transfer, the flow of genetic information between prokaryotes of different species (Syvanen M. 2012), might have occurred between bacterial species and the archaeon. In addition, several other Methanosaeta spp.(such as Methanosaeta thermophila, and other methanogenic archaea (such as Methanosarcina mazei have been found to respond to carboxyl-AHLs and undergo morphological changes in a cell density-dependent manner (Zhang et al. 2012), and so AHL-based quorum sensing may be a universal mechanism in prokaroytes.

Summary

Quorum sensing is a demonstrated process in the archaeon M. harundinacea 6Ac, via the usage of carboxyl-AHLs and the FilR protein, and the changes in gene expression result in the formation of filaments composed of 4 major parts, the cells, spacer plugs, extracellular matrix, and a sheath. As well the filamentous cells have a higher and faster methane output rate, and have decreased protein production, and as a result a lower biomass. These changes enable the archaea to work together as a group and give them an edge over other microbes in the environment, namely being more resistant to unfavourable environment conditions, and being able replicate faster.

References

Beveridge T.J., Patel G.B., Harris B.J. and Dennis Sprott G. “The Ultrastructure of Methanothrix concilii, a Mesophilic Aceticlastic Methanogen.” Canadian Journal of Microbiology, 1986, DOI: 10.1139/m86-128

Davis B.M., Jensen R., Williams P., and O’Shea P. “The Interaction of N-Acylhomoserine Lactone Quorum Sensing Signaling Molecules with Biological Membranes: Implications for Inter-Kingdom Signaling.” PLoS ONE, 2010, 5(10):e13522.doi:10.1371/journal.pone.0013522

Ferry, J.G. “Acetate-Based Methane Production.” Bioenergy, 2008, pp. 155-170

Guo X., Zhang G., Liu X. and Dong X. “Detection of the Quorum Sensing Signals in Methanogenic Archaea.” Acta Microbiologica Sinica, 2011, 51(9):1200-1204

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