Time to abandon the 'progenote' idea?
ouzounis at embl-heidelberg.de
ouzounis at embl-heidelberg.de
Sun Oct 31 16:27:49 EST 1993
In article <1993Oct28.222053.17574 at ac.dal.ca>, arlin at ac.dal.ca writes:
> I'm puzzled as to why every month there appear research articles in
> which the most recent common ancestor of the known groups of cellular
> life is referred to as a "progenote" or "the progenote". For those
> who aren't familiar with this term, a "progenote" is a hypothetical
> type of organism (envisioned by Carl Woese), in which the
> genotype/phenotype relation is inexact [...]
As already pointed out by Dr Hall, the term "progenote" has been used
often in place of the term "progenitor". Personally, I believe that the
"progenote" concept is one of the most powerful guiding principles for
problems of molecular evolution & the origins of life. Unfortunately
it seems that its significance has never been appreciated enough.
The idea of a 'fragmented' genome where genetic processes as we know
them take place in high error rates, represents the most primordial
stage of establishment of those processes. The somewhat mosaic nature
of archaea in terms of genomic composition, between eubacteria and
eukaryotes, argues in favour of the whole concept. The quest for
common genes in all the major domains of Life is just starting. It
is quite remarkable that archaea contain proteins such as histones
and at the same time proteins like HU/IHF (eubacterial nucleoproteins),
or RNA pol B subunits and TFIIB (eukaryotic-like) and at the same
time other eubacterial ribosomal proteins not present in eukaryotes.
Therefore, it seems that the very root of life could have been a very
messy start indeed, what we would probably call a polyphyletic origin
(I am not sure whether this term is applicable to such a deep level).
On the other hand, it is also evident that the progenote, whatever it
looked like, was a rather complex living entity. VERY old proteins,
present throughout life (like glycolytic enzymes) are rather complex
and one cannot possibly imagine that they evolved in parallel to reach
their contemporary complexity level through convergence. Given the slow
rates of change for these molecules, one would expect that forms very
similar to present-day glycolytic enzymes were already present in the
progenote. How these complex proteins were build and how the genes
they coded for were created is one of the greatest biological mysteries.
> Hasn't sufficient evidence accumulated to formally reject this
> hypothesis? Forterre and colleagues [Biosystems (Netherlands) 28(1-3)
> p15-3] have argued persuasively that the existence of homologous
> DNA-polymerase genes with proofreading functions in all groups of
> cellular organisms rules out the possibility that the ancestor had an
> RNA genome or rudimentary error-prone replication. Archaebacteria and
> eubacteria (whose most recent common ancestor would be a common
> ancestor of all known cellular life according to various likely
> phylogenies) share conserved operon structures for RNA Polymerase
> subunit genes, as well as some genes for ribosomal proteins. This [...]
Probably what Woese suggested as the progenote, would represent a
large number of qualitatively different steps of evolution, including
an RNA genome, later a DNA genome, probably still fragmented and perhaps
compartmentalized in a primitive cell membrane. Before attacking a rather
interesting and not well-explored idea, I would give it a further chance.
I would certainly not consider the publication of forterre et al as the
final blow against this hypothesis.
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