David Faguy writes:
> There is a system that is identical functionally and very similar
>structurally in the eubacteria and archaea - the flagellar system.
>However at the molecular level - there is no genetic similarity and in
>fact the molecular mechanisms are quite different.
>> All bacterial flagellins (the protein that make up flagella) are
>homologous. The mechanism of flagellar assembly in bacteria is quite
> unique and appears to be the same in all bacteria.
>> In the archaea flagella perform the same functionand look the same
>but the flagellins are quite distinct - definetly no homology. The
I didn't know there was a method for showing that two proteins are not
homologous, nor was I aware that enough diverse archaebacteria and
eubacteria had been studied to warrant claims that ALL motility
organelles were uniform within each group. In fact, I am not aware that
ANY archaebacterial motor protein/gene has been characterized. Can you
please provide references for these claims? You must be summarizing
alot of new data, since published work (e.g., see Oesterholt and Marwan,
1993, and references therein) would yield a different set of
generalizations:
1. Known eubacterial and archaebacterial flagella are passively rotated
by a membrane-embedded motor, while the analogous eukaryotic motility
organelles (variously called "cilia" "flagella" or "undulipodia")
produce longitudinal waves that are generated *within* the organelle by
microtubule-based sliding-filament motors; rotary motors have not been
observed in eukaryotes; repeated attempts to find micro-tubule based
motility in bacteria (e.g., by Lynn Margulis) have failed.
2. The rotary motors studied in archaebacteria and eubacteria are
driven by ion gradients accross the cell membrane; eukaryotic flagella
are powered by cleavage of energy-rich substrates (I think its GTP, but
don't quote me on this); eubacteria are not "all the same" in this
regard: some use a proton gradient, others use sodium or unknown
cations.
3. Known eubacterial and archaebacterial flagella are external
polymeric filaments composed of one or a few subunits, located outside
the plasma membrane, whereas eukaryotic "flagella" are composed of
hundreds of proteins within the plasma membrane; eubacterial flagellar
filaments may be either left-handed or right-handed;
4. H. halobium (a halophilic archaebacterium) has methyl-accepting
chemotaxis receptor domains that are demonstrably homologous to those of
E. coli and other eubacteria;
5. Scattered attempts to establish homology of some additional motility
components have failed. For instance, eubacterial flagellins and
archaebacterial flagellins cannot be shown to be homologous on the basis
of sequence comparisons. Also, many unsuccessful attempts have been
made over the years to find eukaryotic proteins such as tubulin and
actin in eubacteria and in archaebacteria.
Evidence of the uniqueness and distinctiveness of archaebacterial
motility is not readily apparent, yet David Faguy is not the first to
make such claims. Zillig, et al., 1993 refer to motility just once
in their most recent table comparing features of the three "domains":
eucarya bacteria archaea
flagellins eucaryal bacterial archaeal
That is, they tell us that there are 3 states for the character
"flagellins"! And nothing else is said about motility! By "eucaryal
flagellins" they are presumably referring to proteins that are not
flagellins at all but are called "tubulin," "spoke protein XXX", and so
on. The very limited sense in which this statement is correct is that
archaebacterial and eubacterial flagellins have not been demonstrated to
be homologous, and neither type of flagellin has any demonstrable (or
even suspected) relationship with a eukaryotic protein. This is a
biased, superficial, and unsatisfactory account of the evidence.
Why deny all of the obvious similarities? Does anyone really believe
that gradient-powered rotary flagellar motors and methyl-accepting
chemotaxis proteins evolved twice? Does it hurt so much to admit
that archaebacteria and eubacteria have a common bacterial heritage?
Arlin
refs:
Oesterholt and Marwan, 1993. Signal transduction in halobacteria.
Chapter 5 in _Biochemistry of the Archaea (Archaebacteria)_ (Elsevier,
Amsterdam).
Zillig, et al., 1993. Transcription in archaea. Chapter. 12 in
_Biochemistry of the Archaea (Archaebacteria)_ (Elsevier, Amsterdam).
these references may be difficult to get. if necessary, I can seek out
equivalent references in mainstream journals. write to me. A.S.
