628
Chlamydomonas
several features with those of prokaryotic
organisms: chloroplast ribosomes sedi-
ment at 70S and are sensitive to the same
spectrum of antibiotics as prokaryotic ri-
bosomes. There is considerable sequence
identity between the chloroplast RNAs
and bacterial ribosomal RNAs, and some
chloroplast translation initiation factors
can substitute to some extent for their
E. coli
counterparts
in vitro
. There are, how-
ever, some differences. Although several
chloroplast genes of
C. reinhardtii
have se-
quences resembling the Shine–Dalgarno
sequences preceding the initiation codon,
this is not a general rule. In contrast to
E. coli
, the spacing between the putative
Shine–Dalgarno sequence and the initi-
ation codon is not highly conserved. In
further contrast to
E. coli
, in which most of
the translated messages are polycistronic,
chloroplast genes are usually transcribed
and translated as monocistronic mRNAs
in
C. reinhardtii
.
An unusual feature of
C. reinhardtii
is
that expression of the chloroplast
clpP
gene appears to involve protein splicing
or posttranslational polypeptide ligation.
The
clpP
gene encodes one subunit of the
chloroplast ATP-dependent protease. The
other subunit is encoded by the nuclear
genome. Together, the 5
0
and 3
0
parts of
the
clpP
gene encode a polypeptide that
is homologous and colinear with the
clpP
proteins of other organisms. However, the
858 bp middle part of the
clpP
gene does
not contribute to the 22 kDa
clpP
subunit.
This extra sequence is not excised at the
RNA level because it does not display
structural features typical of chloroplast
introns and because a single transcript
accumulates that includes both the
clpP
coding sequence and the extra sequence.
Therefore, this region is thought to be
removed by protein splicing.
2.3
Mitochondrial Genome
The mitochondrial genome of
C. reinhard-
tii
consists of 15.8-kb linear molecules and
is considerably smaller than the mitochon-
drial genomes of plants, which range be-
tween 200 and 2400 bp. Sequencing of the
entire mitochondrial DNA of
C. reinhardtii
has revealed that it encodes eight protein
genes, three tRNA genes, and two ribo-
somal RNA genes (Fig. 4). The proteins
encoded by the mitochondrial genome of
C. reinhardtii
are cytochrome
b
, subunit
1 of cytochrome oxidase, ±ve subunits of
NADH dehydrogenase, and a reverse tran-
scriptase–like protein whose function is
not yet known. The ends of the mito-
chondrial DNA form an inverted repeat.
The most unusual feature of this DNA
is that the two mitochondrial ribosomal
RNA genes are split into several smaller
coding modules, which are scattered over
nearly half the mitochondrial genome and
are interspersed with protein and tRNA
genes. Expression of these multiple gene
fragments leads to the accumulation of
low- molecular weight subribosomal RNAs
that can base-pair with each other to form
the characteristic secondary structure of
the large and small ribosomal RNAs. The
presence of only three mitochondrial tRNA
genes strongly suggests that the remaining
mitochondrial tRNAs are nucleus-encoded
and imported into the mitochondria, as
has been demonstrated for some plant
mitochondrial tRNAs. Another surprising
feature is that, in marked contrast to other
organisms, the genes for subunits II and
III of cytochrome oxidase and for any of
the subunits of ATP synthetase are not en-
coded by the mitochondrial genome of
C.
reinhardtii
.
None of the mitochondrial genes of
C.
reinhardtii
contain introns. However, the
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