17. Archaea
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Archaea
A domain of life distinct from both Bacteria and Eukarya.
How many domains of life are recognised today?
Three: Bacteria, Archaea and Eukarya.
Who discovered the domain Archaea?
Carl Woese.
What evidence led to the discovery of Archaea?
Comparisons of ribosomal RNA sequences.
Why did Carl Woese use rRNA for phylogenetic analysis?
It is present in all organisms, evolves slowly and contains both conserved and variable regions.
Which rRNA molecule is commonly used to study bacterial and archaeal evolution?
16S rRNA.
What did Carl Woese discover about Archaea?
They are as evolutionarily distinct from Bacteria as either is from Eukarya.
Three-domain system
Bacteria, Archaea and Eukarya.
Are Archaea prokaryotes?
Yes, because they lack a nucleus and membrane-bound organelles.
Why were Archaea originally mistaken for bacteria?
They resemble bacteria morphologically and lack a nucleus.
Are Archaea more closely related to Bacteria or Eukaryotes?
Eukaryotes.
What feature first revealed the evolutionary distinctiveness of Archaea?
rRNA sequence analysis.
Prokaryote
An organism lacking a membrane-bound nucleus.
Eukaryote
An organism with a membrane-bound nucleus and membrane-bound organelles.
Do Archaea have a nucleus?
No.
Do Archaea possess membrane-bound organelles?
No.
What makes Archaea unique?
They combine bacterial-like cellular organisation with many eukaryote-like molecular features.
What type of ribosomes do Archaea possess?
70S ribosomes.
Why are archaeal ribosomes considered bacterial-like?
They are 70S ribosomes similar in size to bacterial ribosomes.
Why are archaeal ribosomes considered eukaryote-like?
Their rRNA, ribosomal proteins and translation factors resemble those of eukaryotes.
Why are many bacterial antibiotics ineffective against Archaea?
Archaeal ribosomes differ sufficiently from bacterial ribosomes to prevent antibiotic binding.
Chloramphenicol
A bacterial protein synthesis inhibitor that targets the 50S ribosomal subunit.
What enzyme activity does chloramphenicol inhibit?
Peptidyl transferase activity.
Why is chloramphenicol ineffective against Archaea?
Archaeal ribosomes differ structurally from bacterial ribosomes.
How is translation initiated in bacteria?
With N-formylmethionine (fMet).
How is translation initiated in Archaea?
With methionine.
How is archaeal translation initiation more similar to eukaryotes?
It uses methionine rather than N-formylmethionine.
What type of bond links membrane lipids to glycerol in Archaea?
Ether bonds.
What type of bond links membrane lipids to glycerol in Bacteria and Eukaryotes?
Ester bonds.
Why are ether bonds advantageous?
They are more chemically stable at extreme temperatures and pH.
What type of hydrocarbon chains are present in archaeal membranes?
Branched isoprenoid chains.
What type of hydrocarbon chains are found in bacterial membranes?
Straight-chain fatty acids.
Why are archaeal membrane lipids distinctive?
They contain ether bonds and branched isoprenoid chains.
What is a tetraether lipid?
A membrane lipid whose hydrocarbon chains span the entire membrane.
What membrane structure can tetraether lipids form?
A lipid monolayer.
Why are lipid monolayers advantageous at high temperatures?
They are more resistant to heat-induced disruption.
Lipid monolayer
A membrane consisting of a single layer of tetraether lipids rather than two leaflets.
Why are archaeal membranes highly stable?
They contain ether bonds, branched chains and often tetraether monolayers.
Do Archaea contain peptidoglycan?
No.
Peptidoglycan
The cell wall polymer characteristic of bacteria.
Why are Archaea resistant to many cell-wall-targeting antibiotics?
They lack peptidoglycan.
Pseudomurein
A peptidoglycan-like cell wall polymer found in some methanogenic archaea.
Which sugars make up pseudomurein?
N-acetylglucosamine and N-acetyltalosaminuronic acid.
What linkage joins sugars in pseudomurein?
β(1→3) linkages.
What linkage joins sugars in bacterial peptidoglycan?
β(1→4) linkages.
Why is pseudomurein resistant to lysozyme?
Lysozyme cannot cleave β(1→3) linkages.
Why is pseudomurein resistant to penicillin?
It lacks the peptidoglycan synthesis pathway targeted by penicillin.
S-layer
A protein or glycoprotein surface layer surrounding many archaeal cells.
Functions of the archaeal S-layer
Provides structural support, maintains shape and protects against osmotic stress.
Most common archaeal cell wall structure
S-layer.
Methanogen
An archaeon that produces methane.
What is one example of an archaeal group containing pseudomurein?
Methanogens.
Asgard archaea
A group of archaea thought to be closely related to the ancestors of eukaryotes.
Why are Asgard archaea important?
They changed our understanding of the evolutionary relationship between Archaea and Eukaryotes.
Lokiarchaeota
An archaeal group first discovered near hydrothermal vents and classified within the Asgard archaea.
Where were Lokiarchaeota first discovered?
Near hydrothermal vents at Loki's Castle. turn60search10
Extremophile
An organism adapted to extreme environmental conditions.
Are all archaea xtremophiles?
No.
Thermophile
An organism adapted to high temperatures.
Psychrophile
An organism adapted to low temperatures.
Halophile
An organism adapted to high-salt environments.
Alkaliphile
An organism adapted to high pH environments.
Why are extreme environments challenging?
They destabilise proteins, membranes and nucleic acids.
Why must thermophiles stabilise proteins?
High temperatures increase molecular motion and promote unfolding.
Why do thermophilic proteins contain more charged amino acids?
To form additional stabilising electrostatic interactions.
Salt bridge
An electrostatic interaction between oppositely charged amino acids.
Why are salt bridges important in thermophiles?
They stabilise protein structure at high temperatures.
Why do thermophilic proteins contain larger hydrophobic cores?
Tighter packing increases protein stability.
Why are thermophilic proteins generally more rigid?
Reduced flexibility helps prevent unfolding at high temperatures.
Major protein adaptations in thermophiles
More salt bridges, more charged amino acids, larger hydrophobic cores and increased rigidity.
Why are thermophilic enzymes often less efficient at low temperatures?
Their structures are too rigid.
Why do psychrophilic proteins contain fewer salt bridges?
To increase flexibility.
Why do psychrophilic proteins have less tightly packed hydrophobic cores?
To remain flexible at low temperatures.
Major protein adaptations in psychrophiles
Increased flexibility, fewer salt bridges and looser hydrophobic packing.
Major trade-off of psychrophilic proteins
They are less thermally stable.
Histone-like proteins
Archaeal proteins that bind and compact DNA.
Why are histone-like proteins important in thermophiles?
They stabilise DNA against heat-induced strand separation.
How do histone-like proteins stabilise DNA?
They compact DNA and reduce strand unwinding.
Reverse gyrase
A DNA topoisomerase found mainly in hyperthermophiles.
What does reverse gyrase do?
Introduces positive supercoils into DNA.
Why is reverse gyrase important?
Positive supercoils make DNA strand separation more difficult.
Compatible solutes
Small organic molecules that accumulate without disrupting cellular processes.
Functions of compatible solutes
Stabilise proteins, stabilise DNA and reduce osmotic stress.
Why are compatible solutes called compatible?
They do not interfere with normal metabolism.
Examples of archaeal compatible solutes
Di-myo-inositol phosphate and mannosylglycerate.
Why do compatible solutes stabilise proteins?
They help maintain proper hydration and folding.
Halomucin
A large glycoprotein capsule produced by Haloquadratum walsbyi.
Function of halomucin
Retains water and reduces desiccation.
Salt-in strategy
Accumulation of KCl to balance external osmotic pressure.
Which archaea commonly use the salt-in strategy?
Extreme halophiles.
Why is KCl accumulated by extreme halophiles?
To prevent water loss by osmosis.
Why is potassium preferred over sodium inside archaeal cells?
It interferes less with enzyme activity.
How do halophiles prevent intracellular sodium accumulation?
By exporting Na+ using membrane transporters.
Compatible solute strategy
Use of organic osmolytes to increase osmolarity without disrupting metabolism.
Examples of compatible solutes in halophiles
Glycine betaine, trehalose and ectoine.
How are halophilic proteins adapted?
They contain more acidic amino acids and fewer hydrophobic surface residues.
Why do halophilic proteins contain more acidic residues?
To increase solubility and maintain hydration in high salt.
What are the major adaptations of thermophilic archaea?
Ether-linked lipids, tetraether monolayers, histone-like proteins, reverse gyrase, stabilised proteins and compatible solutes.
What are the major learning outcomes of this lecture?
Definition of Archaea, archaeal characteristics, and adaptations to extreme environments.