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?