amino-acid, N-terminal extension of this protein.
In parallel, duplication and diversification has
produced a new tRNA Leu/CUN from the gene for
tRNALeu/UUR [60]. All in all, it is clear that multiple rearrangements have taken place in the evolution of the mtDNA so that the order of genes differs quite markedly between the individual animal groups. The tRNA genes are primarily
affected.
The mtDNA genetic codes of the invertebrates
have the same unique features as those of the vertebrates. In addition, AGA and AGG here code
for serine instead of arginine, as in the standard
code, or for STOP in the case of vertebrate
mtDNA. In the mtDNAs of the starfish Pisaster
ochraceus, the sea urchin Strongylocentrous purpuratus and the liver-fluke Fasciola hepatica,
AAA appears to code for asparagine instead of
lysine [24, 331]. The pilgrim scallop, Placopecten
magellanicus, possesses the largest mtDNAs that
have so far been recorded. The insertion of two to
eight copies of a 1442-bp repeat lengthens the
mtDNA to an average of 35 kb and, in extreme
cases, to 42 kb. In various broad-nosed weevils
(Curculionidae), the mtDNA has a total length of
30-36 kb due to increases in an (A + T)-rich
region of 9-13 kb; individual beetles are heteroplastic, with two to five mtDNA size classes [42].
Size variants and heteroplasty, owing to differences in the number of repeats, are also found in
GryUus, various Drosophila species and nematodes [42, 252, 410].
2.9.3 The mtDNA of Ciliates
The mitochondrial DNA of Paramecium aurelia is
a linear double helix with a length of 40469 bp
and deviates markedly from the circular mtDNA
of the Eumetazoa in both gene content and order.
The CO I and II genes of cytochrome oxidase are
present but CO III is not; there are genes for
cytochrome band Nl to N5, but ND4L and ND6
are missing. Instead, there are two other genes
that apparently code for subunits of NADH
dehydrogenase and correspond to parts of the
chloroplast genome. The genes for the ATPase
subunits 6 and 8 are not found, but a gene for
ATPase 9 is present; this has otherwise been
reported only in the mtDNA of yeast and higher
plants. Paramecium mtDNA includes genes for
ribosomal proteins (L2, L14, S12 and S14) that
are usually to be found only in the DNA of plant
mitochondria and chloroplasts and in the mtDNA
of the ciliate Tetrahymena. At least 31 mono- and
2.9.4 Kinetoplast DNA
55
polycistronic transcripts are produced from the
mtDNA of Paramecium but their maturation has
not yet been described. The only partially
sequenced mtDNA of Tetrahymena thermophila
carries genes for CO I, CO II and L14 in the
same order as in Paramecium, but codes for at
least eight tRNAs as against the three tRNAs of
Paramecium. However, there do appear to be differences in the order of the genes between individual Tetrahymena species. The start of replication in Paramecium lies at one end of the
mtDNA, and in Tetrahymena it is in the middle.
The genetic code in the mtDNAs of the ciliates is
in general closer to the standard code than is that
of many other mtDNAs; the translation start
codons here are AUN, GUG and perhaps GUA
[305,351].
2.9.4 Kinetoplast DNA
In stained preparations of trypanosomes and
other flagellates of the group Kinetoplastida, a
body may be seen at the base of the flagellum; it
varies in size from 4-6 !-lm in the genus Trypanosoma to 25-20 !-lm in Crithidia. It is actually a
modified mitochondrium but was at first mistakenly considered to be involved in flagellum movement and was thus termed the kinetoplast. This
cell organelle contains up to 25 % of the total
DNA of these unicellular organisms. The kinetoplast DNA (kDNA) consists of several thousand
smaller, circular DNA molecules (mini-circles)
and up to 50 larger maxi-circles; these mesh with
each other in a 3-D network like the links of a
chain (catemers) [124]. Some strains of Trypanosoma equiperdum are known to lack kinotoplasts.
They have neither a network nor mini-circles, but
contain single circular DNA structures that apparently correspond to the maxi-circles of other
strains [173].
Maxi-circles have a size of 20-40 kb, according
to the species. The length variation stems from
species-specific differences in a non-transcribed
region made up of tandem repeats of various
types. The transcribed region, which corresponds
to the mtDNA of other eukaryotes, has a similar
size of 15-17 kb in all species. The maxi-circles of
Leishmania tarentolae and Trypanosoma brucei
have been completely sequenced. They contain
two rRNA genes, the structural gene for cytochrome b, CO I-III, NDl, ND4, ND5 and several UFRs; ATPase and tRNA genes have not
been found [28, 174]. UUG, CUG and UUA,
which in the standard code all signify leucine,
In parallel, duplication and diversification has
produced a new tRNA Leu/CUN from the gene for
tRNALeu/UUR [60]. All in all, it is clear that multiple rearrangements have taken place in the evolution of the mtDNA so that the order of genes differs quite markedly between the individual animal groups. The tRNA genes are primarily
affected.
The mtDNA genetic codes of the invertebrates
have the same unique features as those of the vertebrates. In addition, AGA and AGG here code
for serine instead of arginine, as in the standard
code, or for STOP in the case of vertebrate
mtDNA. In the mtDNAs of the starfish Pisaster
ochraceus, the sea urchin Strongylocentrous purpuratus and the liver-fluke Fasciola hepatica,
AAA appears to code for asparagine instead of
lysine [24, 331]. The pilgrim scallop, Placopecten
magellanicus, possesses the largest mtDNAs that
have so far been recorded. The insertion of two to
eight copies of a 1442-bp repeat lengthens the
mtDNA to an average of 35 kb and, in extreme
cases, to 42 kb. In various broad-nosed weevils
(Curculionidae), the mtDNA has a total length of
30-36 kb due to increases in an (A + T)-rich
region of 9-13 kb; individual beetles are heteroplastic, with two to five mtDNA size classes [42].
Size variants and heteroplasty, owing to differences in the number of repeats, are also found in
GryUus, various Drosophila species and nematodes [42, 252, 410].
2.9.3 The mtDNA of Ciliates
The mitochondrial DNA of Paramecium aurelia is
a linear double helix with a length of 40469 bp
and deviates markedly from the circular mtDNA
of the Eumetazoa in both gene content and order.
The CO I and II genes of cytochrome oxidase are
present but CO III is not; there are genes for
cytochrome band Nl to N5, but ND4L and ND6
are missing. Instead, there are two other genes
that apparently code for subunits of NADH
dehydrogenase and correspond to parts of the
chloroplast genome. The genes for the ATPase
subunits 6 and 8 are not found, but a gene for
ATPase 9 is present; this has otherwise been
reported only in the mtDNA of yeast and higher
plants. Paramecium mtDNA includes genes for
ribosomal proteins (L2, L14, S12 and S14) that
are usually to be found only in the DNA of plant
mitochondria and chloroplasts and in the mtDNA
of the ciliate Tetrahymena. At least 31 mono- and
2.9.4 Kinetoplast DNA
55
polycistronic transcripts are produced from the
mtDNA of Paramecium but their maturation has
not yet been described. The only partially
sequenced mtDNA of Tetrahymena thermophila
carries genes for CO I, CO II and L14 in the
same order as in Paramecium, but codes for at
least eight tRNAs as against the three tRNAs of
Paramecium. However, there do appear to be differences in the order of the genes between individual Tetrahymena species. The start of replication in Paramecium lies at one end of the
mtDNA, and in Tetrahymena it is in the middle.
The genetic code in the mtDNAs of the ciliates is
in general closer to the standard code than is that
of many other mtDNAs; the translation start
codons here are AUN, GUG and perhaps GUA
[305,351].
2.9.4 Kinetoplast DNA
In stained preparations of trypanosomes and
other flagellates of the group Kinetoplastida, a
body may be seen at the base of the flagellum; it
varies in size from 4-6 !-lm in the genus Trypanosoma to 25-20 !-lm in Crithidia. It is actually a
modified mitochondrium but was at first mistakenly considered to be involved in flagellum movement and was thus termed the kinetoplast. This
cell organelle contains up to 25 % of the total
DNA of these unicellular organisms. The kinetoplast DNA (kDNA) consists of several thousand
smaller, circular DNA molecules (mini-circles)
and up to 50 larger maxi-circles; these mesh with
each other in a 3-D network like the links of a
chain (catemers) [124]. Some strains of Trypanosoma equiperdum are known to lack kinotoplasts.
They have neither a network nor mini-circles, but
contain single circular DNA structures that apparently correspond to the maxi-circles of other
strains [173].
Maxi-circles have a size of 20-40 kb, according
to the species. The length variation stems from
species-specific differences in a non-transcribed
region made up of tandem repeats of various
types. The transcribed region, which corresponds
to the mtDNA of other eukaryotes, has a similar
size of 15-17 kb in all species. The maxi-circles of
Leishmania tarentolae and Trypanosoma brucei
have been completely sequenced. They contain
two rRNA genes, the structural gene for cytochrome b, CO I-III, NDl, ND4, ND5 and several UFRs; ATPase and tRNA genes have not
been found [28, 174]. UUG, CUG and UUA,
which in the standard code all signify leucine,