> "Molecular evidence has now amply confirmed that mitochondria and
> plastids originated as bacterial symbionts. Molecular (particularly
> sequence) data have further pin-pointed those contemporary
> prokaryotes with which mitochondria and plastids are specifically
> affiliated. Two eukaryotic organelles have as their closest living
> relatives the eubacteria (Bacteria _sensu_), with plastids originating
> from within the blue-green algae (Cyanobacteria) and mitochrondria
> from within the alpha-subdivision of the purple bacteria (Proteobacteria).
> It is still unclear whether each organelle is the result of a single
> endosymbiosis of this type (monophyletic origin), or whether each
> arose more than once via prokaryote-eukaryote associations in
> difference eukaryotic lineages (polyphyletic origin). [...]"
> - Michael W. Gray - Origin and evolution of organelle genomes,
> p884, Current Opinion in Genetics and Development 1993 3:844-890.
Mitochondria have about as much in common with bacteria as apples do with
oranges. They both are round fruits, they are about the same size, they
both have DNA/RNA Protein schemes that are similar, They both have many
genes that bear "remarkable similarity", they have histone proteins that
are identical in amino acid sequence, they both grow on trees, they both
can be found in the stomachs of much larger bipedal organisms....
Interesting that those espousing these theories of endosymbiosis do not look
at the differences between bacteria and mitochondria or chloroplasts. How
many of you have ever read a paper on this aspect? What about just for
instance, the presence of two very specialized membranes in the organelles
whereas bacterial membrane is single and also very specialized, but very
different from the organelles. I suppose it would make as much sense to
suggest the nucleus was also an endosymbiont, or maybe that a bacterial cell
infected another bacterial cell, and thus the double membrane, and the
cytoplasm just accumulated in between the two membranes, and thus the origin
of eukaryotes...further infection by other bacteria infected by other
bacteria, resulted in the double membraned organelles. (This theory is
copyrighted, just in case it catches on).
While neither I nor anyone else on this planet has a clue yet why
mitochondria make a few proteins on their own, the answer may lie in the
specialized nature of these proteins. Most of them form subunits of larger
polymeric proteins, whose other subunits are cytoplasmic proteins encoded
for in the cell nucleus. Without, for the moment worrying about how such a
feature could be orchestrated between an endosymbiont and its host, lets
look at what is involved in getting these cytoplamic components into the
mitochondrion.
There are some major hurdles to getting proteins into the organelle. The
specialized proteins produced in the cytoplasm are targeted for either the
inner or outer compartments of the mitochondrion (or the additional third
thylakoid space in chloroplasts) or for the inner or outer membranes of the
organelle. The development and growth of the mitochondrion is a major feat,
in which proteins targeted for any of these four or five spaces, must be
designed to get to the proper place (the membranes do not allow proteins to
pass). Special signal peptides specific for the destination are included in
the proteins synthesized for the mitochondria in the cytosol. Specific
receptors in the various locations of the organelles allow the proteins to
get to their appropriate destinations, an incredible feat. Nobody has a
clue how the nucleus knows how much protein to make to match up with the
mitochondrion production, but the two processes are coordinated. Perhaps
the reason why some proteins are produced in the inner membrane may be that
they could not be imported, or that they would interact in the cytoplasm
with the proteins they interact with in the matrix of the organelle, and
thus would pervent the passage into the organelle.
One question we might want to ask the endosimbiont believers is the same
question I posed above: why do the organelles make any proteins at all? why
not transfer all of the protein synthesizing process to the nucleus? The
nucleus carries about 100 genes specifically required just for the organelle
to be able to synthesize the proteins that it makes, an exceedingly costly
arrangement. To quote from Alberts, et. al. "The Molecular Biology of the Cell"
"The reason for this costly arrangement is not clear, and the hope
that the nucleotide sequence of the mitochondrial and chloroplast genomes
would provide the answer has proved unfounded. We cannot think of
compelling reasons why the proteins made in mitochondria and chloroplasts
should be made there rather than in the cytosol.
[This whole problem] is difficult to explain by any hypothesis that
postulates a specific evolutionary advantage of presentday mitochondrial or
chloroplast genetic systems." p715.
Art
http://chadwicka.swac.edu