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278 PLASTIDS, MITOCHONDRIA, AND HYDROGENOSOMES
is, they divide independently of the host cell,
often existing as multicopy structures in each
eukaryotic cell. Several of the same proteins
responsible for the division process in bacteria
(FtsZ, MinD, MinE) also participate in mitochondrial
and plastid fission. Ultrastructurally,
mitochondria and plastids resemble bacteria
in select ways. The organelles are bounded by
two membranes, which are likely homologous
to the plasma membrane and outer membrane
of the Gram-negative ancestors from which
they derive. The inner membrane of both
mitochondria and plastids is highly convoluted,
forming cristae in mitochondria and
thylakoids in photosynthetic plastids. These
convolutions are undoubtedly adaptations
to expanding the surface area for the major
processes within the organelle, oxidative respiration
in mitochondria and photosynthesis
in plastids. No such folding of the inner membranes
occurs in the prokaryotes thought to
be ancestral to mitochondria, but inner membrane
convolutions homologous to plastid
thylakoids are seen in photosynthetic bacteria
(the likely ancestors of plastids). It is abundantly
clear that the metabolisms of the
two organelles are descended from ancestral
prokaryotic processes. The Krebs cycle and
oxidative phosphorylation in mitochondria
are virtually identical to the same pathways in
bacteria, with most of the enzymes, cofactors,
electron carriers and ATP synthases sharing
the same evolutionary heritage. Similarly, the
engines of photosynthesis in plant and algal
plastids are plainly derived from the machinery
of cyanobacteria-like prokaryotes.
Perhaps the most persuasive line of evidence
for the endosymbiotic origin of mitochondria
and plastids are the organelles’ genomes. Both
organelles have their own DNA, and the architecture
of their genome is classically prokaryotic,
being circular and having a single origin
of replication and genes arranged in operons
like prokaryotes. This organization is in stark
contrast to that of the host nucleus with multiple,
linear chromosomes and individual genes,
each with its own regulatory elements and a
single gene transcript. The organization of mitochondrial
and plastid gene operons also reflects
prokaryotic ancestry, with similar arrangements
of genes. It is also clear that the organelles’
genes are closely related to bacterial genes.
Indeed, gene trees provide us with a clear
picture of organelle evolution. Mitochondrial
sequences are most closely related to those
of alpha-proteobacteria, whereas plastid
sequences are most closely related to those of
cyanobacteria. The gene trees tell us several
important things. Firstly, these organelles
must have arisen from separate endosymbiotic
events: one for the mitochondria and one
for the plastids, because their genes obviously
derive from different parts of the bacterial
radiation. If both mitochondria and plastids
emerged in one place from the bacterial tree, we
might assume that they derived from a common
endosymbiosis, but this is not so. Secondly,
the alliances between mitochondria and alphaproteobacteria,
and between plastids and
cyanobacteria, rationalizes the similarities we
observe in their metabolisms. Cyanobacteria
are photosynthetic. Alpha-proteobacteria possess
a Krebs cycle and oxidative phosphorylation,
although these respiratory processes occur
widely in prokaryotes and are not restricted
to alpha-proteobacteria. Further down the
information chain we also see identity between
mitochondria and plastids and prokaryotes.
The transcription of RNA from DNA utilizes the
same type of RNA polymerase components,
although select mitochondria and plastids also
appear to have recruited a viral RNA polymerase
as well. The translation of protein from mRNA
in mitochondria and plastids is also prokaryotic,
using Shine–Dalgarno ribosome binding
motifs, formyl methionine as the initiator
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA