Time to abandon the 'progenote' idea?
Sat Oct 30 14:48:23 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-- modern cellular organisms are
>all "genotes". Progenotic replication, transcription, and translation
>are rudimentary and error-prone, which necessitates small genes and
>small segmented genomes, according to Woese (e.g., see "Bacterial
>Evolution", Microbiol. Rev. 51: 221-271.). Woese argued that
>progenotes must have been a necessary stage in the evolution of modern
>cellular life, and hypothesized that the ancestor of eubacteria,
>eukaryotes and archaebacteria was in fact such a progenote, even that
>this organism might have had an RNA genome.
>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
>indicates that their most recent common ancestor must have had long
>DNA genes (i.e., not short gene-segments) arranged in operons (wow!).
>At least a dozen ribosomal proteins (as well as some translation
>factors) are shared by all cellular organisms [Wittmann-Liebold, et
>al., in _The Ribosome: Structure, Function and Evolution_, ed. W.
>Hill, et al. (American Society for Microbiology Press, Washington DC,
>1990)], indicating that their most recent common ancestor must have
>had a rather complex polyfunctional (i.e., not 'rudimentary')
>ribosome. What gives?
I think Barry Hall is right, most scientists use the
term progenote incorrectly to mean progenitor.
The many shared characteristics of all extant life forms and
the many ancient duplicated genes that apparently already
had been duplicated in the last common ancestor (ATPases,
elongation factors, dehydrogenases, met-tRNAs, glutamine
synthases, glutamate dehydrogenases, heat shock protein
analogs....), paint a very different picture of the last common
ancestor than a primitive progenote (as originally
defined). Many of the ancient duplicated genes result in
molecular phylogenies with a similar topology to the
16S rRNA tree. It appears unlikely that these were
spread later during the evolution by means of horizontal
gene transfer. To the contrary, we can recognize cases of
horizontal gene transfer against the background of genes
that show the consensus phylogeny.
However, there are still scientist who argue strongly in
favor of the progenote not only as the last common ancestor,
but of a progenote-like phase reaching even further up the
tree (i.e. up to the three domain or Urkingdoms
ancestors. e.g. Kandler, Zillig). I think they are wrong,
but they certainly prove that the progenote idea is not dead.
[Zillig, W., Palm, P. and Klenk, H-P.: 1992. in: The origin and
evolution of the cell (H Hartman, K Matsuno, eds.) World
Scientific, Singapore, New Jersey, London, Hong Kong, pp.
Kandler, O.: 1993, in: Early Life on Earth: Nobel Symposium No.
84. S. Bengston (ed) Columbia University Press, New York,
Gogarten, J.P. Starke, T., Kibak, H., Fichmann, J. and Taiz,
L.: 1992, J. Exp. Biol. 172, 137-147
Gogarten, J.P. and Taiz, L.: Rooting the tree of life. 1992,
Photosynth. Res., 33, 137-146]
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