Metabacteria

[Ken Kinman]

Hi Brian,
      Yes, I am aware of the "lipid problem".  I strongly suspect that the eukaryotic host cell evolved from something intermediate between posibacteria and metabacteria ("archaebacteria"), which had evolved many metabacterial features but not yet switched to ether-linked lipid membranes in a major way.  They would no doubt cladistically fall out as primitive metabacteria, but whether such forms are still extant is uncertain.  There are lots of metabacteria out there which are uncultured (only known from environmental sequences) or just undiscovered, so who knows.  Or such forms might have gone extinct long ago (like pelycosaurs and theraspids that gave rise to extant mammals).  Anyway, I think there was probably a very long transition during which ether-linked lipids gradually replaced ester-linked lipids (similar to the slow migration of bones from the jaw into the mammalian ear, and one jaw articulation slowly replacing the other).  In any case, this is how I tend to approach the lipid problem.
  ---Cheers,
         Ken Kinman
P.S.  Another possibility (although less likely, in my opinion) is that a eubacterium was the eukaryotic host cell which engulfed a metabacterial "proto-nucleus".  That would take care of the lipid problem even more easily.  Too bad we don't have lots of prokaryotic fossils to solve these problems, the way we did with mammalian origins.
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B.J.Tindall wrote:

Ken,
To which my answer is, if the Archaebacteria/Archaea/Metabacteria are the ancestors of the eukaryotes why don't eukaryotes have isoprenoid ether linked lipids? Secondly the stereo chemistry of the lipids in eubacteria + eukarya is different to that in Archaebacteria/Archaea/Metabacteria. Yoshe Koga's group have shown that one of the key enzymes in the synthesis of the isoprenoid ether linked lipids vs the synthesis of ester linked lipids is not only stereospecific, but the two enzymes probably share not common evolutionary origin. Thirdly the properties of the acyl vs isoprenoid based memberanes are significantly different. Most people are either not aware of that or simply sweep these facts under the carpet.....