Génomique comparative

Parasites go the full monty
PATRICK J. KEELING, Nature 414, 401–402 (22 November 2001)

Parasites are well known for stripping themselves down to the bare essentials, 
but how far will they go? Microsporidia are one group of single-celled parasites 
that have taken it to extremes and gone the 'full monty'. On page 450 of this issue1,
 Katinka and colleagues expose the shocking extent of this striptease in the complete
 genome sequence of the microsporidian Encephalitozoon cuniculi , which parasitizes 
a range of mammals, including humans. This genome is a mere shadow of those found in
 other eukaryotes (organisms with nucleated cells), as it consists of only 2.9 million
 base pairs — less than 0.1% the size of the human genome. This is even smaller than 
the genomes of many bacteria. Nevertheless, the genome is a treasure trove of 
information on the powerful reductive forces that shape these unusual parasites.
....
Microsporidia, however, have invented a unique system of core energy metabolism. 
They use parts of typical mitochondrial metabolic pathways, mixed with pathways found 
in other organisms that lack mitochondria, resulting in a system that is fundamentally
 different from either1, 9 . In particular, microsporidia lack pyruvate:ferredoxin 
oxidoreductase — a key enzyme in other organisms that lack mitochondria, and a common 
target for anti-parasitic drugs — and use mitochondrial pyruvate dehydrogenase in its 
place1, 9. Once again, we see François Jacob's image of evolution as a tinkerer10 , 
using bits and pieces of existing machines to build a completely new one — in this case,
 something as central to life as a new form of energy metabolism.

Do all organisms use the same gene for the same purpose? 
The answer here is clearly "no". 
 Example: 
   2.When the genome of the archaean Methanococcus jannaschii was sequenced, four of the 20 amino-acyl-tRNA
     synthetases could not be found. 
   3.These enzymes are critical in preparing the tRNAs for protein synthesis. 
   4.Biochemical evidence indicated that glutamine and asparagine are incorporated as transamindated derivatives of
     glutamate and aspartate, so the absence of these tRNA synthetases was not surprising. 
   5.A comparable story was postulated for cysteine, hypothesizing that cysteine-tRNA is produced by trans-sulfuration
     of serine-tRNA. 
   6.But the absence of lysyl tRNA synthetase was unexplained. 
   7.Subsequently the lysine-tRNA synthetase was identified -- it is dissimilar to the other tRNA syntetases, apparently a
     new class. 
   8.Not the product of rapid evolution. 
   9.The bacteriuim Borrelia burgdorferi was subsequently found to have the same kind of lysine-tRNA synthetase as
     that found in M. jannaschii 
  10.Likely explanations: 
     1.Parallel evolution 
     2.Horizontal gene transfer & substitution