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PLASTIDS 287
the domain of botany and algology. No more!
Plastids have been identified in apicomplexan
parasites in recent years. Researchers have
gathered a host of biological, biochemical and
molecular information from malarial and
toxoplasmodial parasites. However, by focusing
on them as parasites, and the effects they
have on humans as the host, we missed a key
feature; these parasites started life as autotrophic,
photosynthetic, alga-like organisms. The
key to this revelation was the identification of
a plastid, a small parasite organelle that shares
the same evolutionary heritage as chloroplasts
of plants and algae.
Just as the theory of endosymbiosis explains
the origin of mitochondria from alphaproteobacteria,
it also explains the origin of
a second organelle, the plastid, or chloroplast.
Plastids clearly originated from engulfed
cyanobacteria, which brought the power of
photosynthesis into eukaryotes. Whereas the
driver for mitochondrial endosymbiosis is
posited to have been respiration or hydrogen
syntrophy, in plastid origins it is argued to
have been autotrophy that drove the partnership.
Plastid acquisition followed mitochondrial
acquisition and created the eukaryotic
autotrophs. Reduction of the cyanobacteria-like
endosymbiont followed a similar course to the
reduction of the mitochondrial endosymbiont
with transfer of many cyanobacterial genes to
the host nucleus. Similar to nucleus-encoded
mitochondrial gene products, the plastid
proteins encoded by genes relocated to the
nucleus must also be targeted to the plastid to
function normally. These plastid proteins are
also targeted using N-terminal extensions distinct
from those of mitochondrial proteins.
Once established this system again allowed
wholesale intracellular gene transfer, and the
great majority of genes for plastid proteins are
now located in the nucleus. The residue, typically
in the order of 100 to 200 genes in plastid
DNA, is clearly cyanobacterial in origin with
a circular architecture, a single origin of replication,
genes in ancestral operons, 10, 35
promoters for the bacterial type RNA polymerase,
and an absence of spliceosomal introns
(though bacterial-like group I and group II
introns occur in plastids). Thus, the typical plant
or algal cell has three genomes, the mitochondrial,
the plastid, and the nucleus, the latter
also containing a large number of genes
acquired from the organelles.
A mysterious genome
US researcher Araxie Kilejian started the trail
of research clues that led to the discovery of a
plastid in malaria. She used EM to examine a
circular, extrachromosomal DNA molecule in
Plasmodium lophurae, a malarial parasite of
birds. Not expecting to see plastid DNA in a parasite,
Kilejian naturally assumed these DNA circles
were the parasite’s mitochondrial genome.
Similar circles were found in other malarial
parasites and Iain Wilson’s group in London
commenced a study of the 35 kb circle from
Plasmodium falciparum, which causes cerebral
malaria in humans. The first sequence
data from the circular genome indicated that
the genes were prokaryotic in nature. Ironically,
finding bacterial-type genes on the 35 kb circle
only reinforced the misconception that it represented
the mitochondrial genome, because
mitochondrial DNAs are typically circular and
harbor genes of bacterial origin. But when
a third parasite genome was discovered, the
penny dropped. The third genome, a linear
6–7 kb element existing as tandem repeats,
carried mitochondrial-type cytochrome genes
(see above), and in subcellular fractionations,
only the linear element co-purifies with mitochondria.
The 35 kb genome wasn’t the mitochondrial
DNA; alternative explanations had
to be considered.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA