ARCHAEA

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ARCHAEA

• THE THIRD KINGDOM OF LIFE
• See Prescott 5th Edn., Ch 20.

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The Universal Phylogenetic Tree of Life
BACTERIA
ARCHAEA
EUKARYOTES
LUCA (Last Universal Common Ancestor
Progenote (Origin of Life gt3.85Bn years ago)
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What are Archaea?

• Archaea are
• Prokaryotes
• Single cell microorganisms
• Morphologically like bacteria (rods, cocci and
  other forms)
• Lack a nuclear membrane
• Physiologically diverse (like bacteria) and
  include chemoautotrophs, heterotrophs
  (organotrophs) aerobes and anaerobes
  psychrophiles, mesophiles and thermophiles
• At a molecular level, Archaea have structures and
  systems which are sometimes bacteria-like,
  sometimes eukaryote-like and sometimes unique.

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Unique Archaeal features 1

• Cell wall
• Either protein S-layer (stain gm-ve), or
• Pseudomurien
• No peptidoglycan as in bacteria (no muramic acid,
  no D-amino acids
• Cell membrane
• C40 Biphytanyl ether linked lipid monolayer
  (c.f., C16-18 fatty acyl glycerol lipid bilayers)

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Unique Archaeal features 2

• Methanogenesis
• The synthesis of methane from H2 and CO2.
• CO2 4H2 ? CH4 2H2O
• Uses a complex pathway with unique cofactors (not
  found in any other organism)
• TCA Cycle
• Some Archaea lack a full TCA cycle, and use a
  reductive (reversed) TCA cycle (with ATP and
  NADH) to fix CO2 and synthesise acetyl CoA.
• Glycolysis
• Glucose is not degraded by the Embden Meyerhof
  pathway
• Archaea use a modified Entner-Douderoff Pathway

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Bacterial features of Archaea

• Morphology and size
• Lifestyle and metabolism
• Circular ds DNA chromosome (0.5 3 Mbp)
• mRNA similar
• Promoters similar
• No introns
• 5S, 16S ribosomal subunits

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Eukaryotic features of Archaea

• Most similarities are in DNA processing
• DNA-dependent RNA polymerases have same
  multi-subunit structures as in eukaryotes
• Initiator tRNA carried Met (so first amino acid
  in every translated protein is Met)
• Elongation factor EF-2 reacts with diptheria
  toxin
• Some archaea have histone-like proteins

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Archaeal Phyla

• Archaea are divided into 3 or 4 phyla
• Crenarchaeotes (including some hyperthermophiles,
  and largely unknown marine and terrestrial
  mesophiles and psychrophiles
• Euryarchaeotes (including all methanogens,
  extreme halophiles, sulphate reducers, and the
  S-dependent thermoacidophiles)

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Summary of Archaeal Groups 1
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Summary of Archaeal Groups 2