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Mitosomes rewrite evolutionary theory Giardia intestinalis possesses highly reduced mitochondria-
derived = organelles By David Secko
The evolutionary transition from prokaryotes to eukaryotes-characterized = by eukaryotic cells
obtaining a nucleus and two membrane compartments-is = still subject to considerable debate. It has
been suggested that a = nucleus developed first, followed by the acquisition of mitochondria, a =
theory that has partly relied on the single-celled eukaryote Giardia = intestinalis because it has a
nucleus and no mitochondria, suggestive of = an intermediate step. The recent identification of
mitochondrial = remnants in other amitochondrial protists suggests this may not be true. = In the
November 13 Nature, Jorge Tovar and colleagues at Royal Holloway, = University of London report that
Giardia intestinalis possesses highly = reduced mitochondria remnants called mitosomes (Nature, 426:172-
176, = November 13, 2003).
The work was stimulated by the cloning of a Giardia gene known in other = organisms to be involved
in the biosynthesis of Fe-S clusters, a = critical function of the mitochondria. It was the second
such gene to be = identified in Giardia (the proteins of which are named IscU and IscS), =
suggesting the potential for a mitochondrion-derived organelle. Tovar et = al. generated antibodies
to IscU and IscS and demonstrated that both = proteins were present in a high-speed centrifugation
pellet containing = membranes and membrane-bound organelles, but not in the cytosol. This = high-
speed sediment was also capable of assembling Fe-S clusters in = vitro, signifying the presence of
this mitochondrial biosynthetic = pathway in the amitochondrial Giardia cells.
They then attempted to identify the cellular location of the IscU and = IscS. Confocal scanning
fluorescence microscopy and anti-IscS and = anti-IscU antibodies suggested that the proteins were
localized to small = cellular structures distributed throughout the cytoplasm. A more = detailed
view of these structures was provided by immunoelectron = microscopy, which localized IscS and IscU
to these small organelles and = revealed them to be "bounded by two limiting membranes," a
distinctive = feature of mitochondria (since Giardia contains no chloroplast-related =
biochemistry).
"Our data provide direct physical evidence for the presence of = mitochondrial remnant organelles
(mitosomes) in Giardia, as postulated = on the basis of previous phylogenetic and biochemical
studies," conclude = the authors.
In an accompanying preview article, Katrin Henze and William Martin from = the University of
Dusseldorf comment, "These findings mark a turning = point for views of early eukaryotic and
mitochondrial evolution: = Giardia's place as an intermediate stage in standard schemes of =
eukaryotic evolutionary history is no longer tenable."
Read the rest at The Scientist.com biomedcentral.com02Open ↗
Comment: Lynn Margulis has done foundational work on symbiogenesis.
Kind Regards, Robert Karl Stonjek. ------=_NextPart_000_0063_01C3AA91.552258C0 Content-Type:
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<DIV><FONT face=3D"Courier New" size=3D2>Mitosomes rewrite evolutionary=20 theory</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2>Giardia intestinalis possesses = highly=20 reduced mitochondria-
derived organelles</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2>By David Secko</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>The evolutionary transition = from=20 prokaryotes to
eukaryotes=97characterized by eukaryotic cells obtaining = a nucleus=20 and two membrane
compartments=97is still subject to considerable debate. = It has=20 been suggested that a
nucleus developed first, followed by the = acquisition of=20 mitochondria, a theory that has
partly relied on the single-celled = eukaryote=20 Giardia intestinalis because it has a nucleus
and no mitochondria, = suggestive of=20 an intermediate step. The recent identification of
mitochondrial = remnants in=20 other amitochondrial protists suggests this may not be true. In
the = November 13=20 Nature, Jorge Tovar and colleagues at Royal Holloway, University of =
London=20 report that Giardia intestinalis possesses highly reduced mitochondria = remnants=20
called mitosomes (Nature, 426:172-176, November 13, 2003).</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>The work was stimulated by the = cloning of a=20 Giardia
gene known in other organisms to be involved in the biosynthesis = of Fe-S=20 clusters, a
critical function of the mitochondria. It was the second = such gene=20 to be identified in
Giardia (the proteins of which are named IscU and = IscS),=20 suggesting the potential for a
mitochondrion-derived organelle. Tovar et = al.=20 generated antibodies to IscU and IscS and
demonstrated that both = proteins were=20 present in a high-speed centrifugation pellet
containing membranes and=20 membrane-bound organelles, but not in the cytosol. This high-speed
= sediment was=20 also capable of assembling Fe-S clusters in vitro, signifying the = presence
of=20 this mitochondrial biosynthetic pathway in the amitochondrial Giardia=20
cells.</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>They then attempted to identify = the=20 cellular location
of the IscU and IscS. Confocal scanning fluorescence=20 microscopy and anti-IscS and anti-IscU
antibodies suggested that the = proteins=20 were localized to small cellular structures
distributed throughout the=20 cytoplasm. A more detailed view of these structures was provided
by=20 immunoelectron microscopy, which localized IscS and IscU to these small=20 organelles and
revealed them to be =93bounded by two limiting = membranes,=94 a=20 distinctive feature of
mitochondria (since Giardia contains no=20 chloroplast-related biochemistry).</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>=93Our data provide direct = physical evidence=20 for the
presence of mitochondrial remnant organelles (mitosomes) in = Giardia, as=20 postulated on the
basis of previous phylogenetic and biochemical = studies,=94=20 conclude the
authors.</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>In an accompanying preview = article, Katrin=20 Henze and
William Martin from the University of Dusseldorf comment, =
=93These=20
findings mark a turning point for views of early eukaryotic and = mitochondrial=20 evolution:
Giardia's place as an intermediate stage in standard schemes = of=20 eukaryotic evolutionary history
is no longer = tenable.=94<BR></FONT></DIV>
<><FONT face=3D"Courier New" size=3D2>Read the rest at The=20 Scientist.com</FONT></DIV>
<><A href=3D"biomedcentral.com02Open ↗"><FONT=20 face=3D"Courier New"=20
size=3D2>biomedcentral.com02Open ↗</FONT></A></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>Comment:</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2>Lynn Margulis has done = foundational work on=20
symbiogenesis.</FONT></DIV>
<><FONT face=3D"Courier New" size=3D2></FONT> </DIV>
<><FONT face=3D"Courier New" size=3D2>Kind Regards,<BR>Robert Karl=20
Stonjek.</FONT></DIV></BODY></HTML>
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