Daily Newsletter March 21, 2012

Daily Newsletter March 21, 2012

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Today's Topic: Archaea

The Archaea represent the third domain of life, and as such posses characteristics that make them different from either Bacteria or Eukarya. They are prokaryotic, but with some unique features. Your goal today is to learn what makes Archaea different from Bacteria.

You may have heard in the past that Archaea are extremophiles, living only in extreme environments. This is a lovely fiction. While many Archaea live in extreme environments, it is not a requirement. Some even live in moderate environments. So what then makes them different?

First, they are prokaryote, which makes them different from Eukarya (there is more than just this, but let's wait for a moment). Like Bacteria, they have cell walls, but their cell walls are not based on peptidoglycan. Instead, the Archaea form an S-Layer to act as the cell wall.

The Archaea produce transmembranal proteins to create a schaffold for globular proteins.  These globular proteins are then arraied to form a protective coating similar to "chain mail" (see the image below).  Some Archaea, such as members of the Order Methanobacteriales add pseudomurein to add additional support and strength to their cell walls.  Pseudomurein is similar to peptidoglycan, but lacks the ordered arrangement of glycan chains.

Another major difference between the Archaea and both the Eukarya and Bacteria is in the formation of their cell membranes.  You will recall discussions of phospholipids and cell membranes.  You may remember that we always discuss phospholipids as having ester linkages between fatty acids and the gylcerol backbone.  This is not the case with Archaea.  Archaea use an ether linkage between fatty acids and the glycerol backbone.  In addition, archaeal fatty acids can be connected across the membrane, resulting in monolayers (instead of bilayers).  A molecule with two polar heads seperated by a non-polar region is known as a bolaamphiphile.

Consider the Archaeal phospholipid:  What advantage would an ether linkage provide?  What advantage would a bolaamphiphile provide?  Would you make these the same way you would make a bacterial phosophlipid?  What type of metabolic pathways would you have to have?  Based on this physiological difference, can you compare the evolutionary history of these two organisms? 

You will also note in the above diagram that the fatty acids are also different.  Archaeal fatty acids are based on isoprenoids and they can have cyclopropane and cyclohexane ring structures.  While there are other differences to Archaeal phospholipids, digest on what we have mentioned for now.  One thing to note:  while the chemical architecture may be different, Archaeal phospholipids serve the same function as other phospholipids, and their membranes have the same function as other cellular membranes.

Unlike Bacteria, but like Eukarya, Archaea use Methionine on the initator tRNA.  There are also some structural differences between the Archaeal tRNA, and those found in Bacteria and Eukarya.  As with bacteria, Archaea lack introns, and do not undergo splicing. 

They have 70S ribosomes, but their ribsomes are different enough from Bacteria that the ribosome effecting antibiotic chlramphenicol does not affect Archeal ribosomes.  Conversely, the Eukaryotic ribosomal toxin anisomycin also affects Archaeal ribosomes.  So while they are bacterial in size, they so similarities to the Eukaryotic ribosome.

Another anomylous protein is DNA-dependent RNA polymerase (RNA polymerase that makes mRNA).  While it is a single enzyme, like Bacteria, it is a complex protein that more closely resembles the subunit patterns of Eukaryotic DNA-Dependent RNA Polymerase.  Also, the antibacterial compound Rifampicin, which blocks mRNA transcription in bacteria, does not work on either Archaea or Eukarya.  Arhaea also have RNA polymerase II type promoters, just like Eukarya.

Archaea are neither Bacterial or Eukaryal, but they share some characteristics.  They also have some unique characteristics. 

Archaea have a single circular genophore (molecule of DNA), just like Bacteria.  They also have a process analogous to conjugation for horizontal gene transfer.  It has been shown though that Archaea possess histones, and while they don't function just like Eukaryal histones, they share a common ancestry.

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Daily Challenge: Archaeal Characteristics
In your own words, describe the general characteristics of the Archaea. It has been hypothesized in the past that the Archaea are related to the Proto-Eukaryotic cell. What is your view on that with what you have read? Justify your answer.