Plant Molecular Biology 36: 285–295, 1998.
285
c
1998 Kluwer Academic Publishers. Printed in Belgium.
Complete sequence of the mitochondrial DNA of Chlamydomonas eugametos
Eileen M. Denovan-Wright, Aurora M. Nedelcu and Robert W. Lee
Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada (
author for
correspondence)
Received 18 June 1997; accepted in revised form 17 September 1997
Key words: Chlamydomonas eugametos, evolution, genome sequence, green algae, group I introns, mitochondrial
DNA
Abstract
The completenucleotidesequence of the Chlamydomonaseugametos(Chlamydomonadales,Chlorophyceae,sensu
Mattox and Stewart) mitochondrial genome has been determined (22 897 bp, 34.6% G
+
C). The genes identified
in this circular-mapping genome include those for apocytochrome b, subunit 1 of the cytochrome oxidase complex,
subunits 1, 2, 4, 5, and 6 of the NADH dehydrogenase complex, discontinuous large and small subunit ribosomal
rRNAs and three tRNAs whose anticodons CAU, CCA and UUG are specific for methionine, tryptophan and
glutamine, respectively. TheC. eugametosmitochondrialDNA (mtDNA), therefore, sharesalmost the same reduced
set of coding functions and similar unusual features of rRNA gene organization with the linear 15.8 kb mtDNA
of Chlamydomonas reinhardtii, the only other completely sequenced chlamydomonadalean mtDNA. However,
sequence analysis of the C. eugametos mtDNA has revealed the following distinguishing features relative to those
of C. reinhardtii: (1) the absence of a reverse transcriptase-like gene homologue, (2) the presence of an additional
gene for tRNA
met
that may be a pseudogene, (3) a completely different gene order, (4) transcription of all genes from
the same mtDNA strand, (5) a lower G
+
C content, (6) less pronounced bias in codon usage, and (7) nine group I
introns, several of which contain open reading frames coding for potential maturases/endonucleases and two have a
nucleotide at the 5
0
or 3
0
splice site of the deduced precursor RNAs that deviates from highly conserved nucleotides
reported in other group I introns. The features of mitochondrial genome organization and gene content shared by
C. eugametos and C. reinhardtii contrast with those of other green algal mtDNAs that have been characterized in
detail. The deep evolutionary divergence between these two Chlamydomonas taxa within the Chlamydomonadales
suggests that their shared features of mitochondrial genome organization evolved prior to the origin of this group.
Introduction
Current knowledge about the gene content, structure
and phylogenetic affiliations of mitochondrial gen-
omes from the green algal order Chlamydomonadales,
class Chlorophyceae, sensu Mattox and Stewart [35],
when compared with those of other green algal taxa
and embryophytes (land plants), raises fundament-
al issues about the evolutionary strategies undertaken
by the mitochondrial lineages within the green algal
group [21, 23, 30, 51]. Available information for
The nucleotide sequence data reported will appear in the EMBL,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession number AF008237.
the green algae suggests a rather unexpected dicho-
tomy in mitochondrial genome organization, which
is either ‘animal-like’ in chlamydomonadalean taxa
or ‘plant-like’ in Prototheca wickerhamii and Platy-
monas (Tetraselmis) subcordiformis. The best studied
mitochondrial genome from the Chlamydomonadales
is that of Chlamydomonas reinhardtii (for reviews,
see [10, 22, 37]). The 15.8 kb linear molecule codes
for eight proteins and three tRNAs as well as discon-
tinuous small (SSU) and large subunit (LSU) rRNAs.
The rRNA-coding regions are fragmented into subgen-
ic modules that are interspersed with each other and
with protein-coding and tRNA genes [7]. No introns
have been detected in any of the coding regions of the