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mito carriers and evol. split

David R. Nelson dnelson at utmem1.utmem.edu
Mon Aug 5 17:22:24 EST 1996

	In looking at an alignment of about 130 mitochondrial carriers
(http://drnelson.utmem.edu/nelsonhomepage.html) I have found one conserved
motif that is repeated in these proteins that is relevant to the divergence
of lower eukaryotes, plants and animals.  This is a position within the
mitochondrial carrier signature motif P(hydrophobic)(D,E)XX(K,R).  This
motif occurs 3 times in each carrier, reflecting an early gene triplication
to form the common ancestor of these proteins.  Pro is seen in 95% of 352
occurrences of this motif, however, it is replaced by S in 15 sequences in
the second domain.  It seems logical to assume that P was the ancestral
condition and S occured later.
All 15 sequences are either plant sequences or lower eukaryotes like yeast,
Chlamydomonas, Neurospora and one trypanosome.  The remainder of the
sequences are animal, with two Plasmodium sequences and one Chlorella

	This seems to suggest that S appeared in the second domain of these
carriers, only in a lineage that gave rise to plants and lower eukaryotes.
Animals appear to have retained the P at this position.  


                     S  |------ plants 
              P   |-----|
         ---------|     |------ lower eukaryotes
                  |------------ animals

This does not fit with the tree of life homepage that puts plants splitting
first before animals and fungi diverged.  


I would be interested in your comments on this discrepancy.  Just how
firmly is this divergence established?  If it is well established, how can
I explain this observation?

David R. Nelson
Assistant Professor
Dept. of Biochemistry
University of Tennessee, Memphis

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