plant mitochondria - senecio7の日記

Kubo T & Newton KJ 2008 Angiosperm mitochondrial genomes and mutations. Mitochondrion 8:5-14.

the sizes of angiosperm mitochondrial genomes have expanded since plants colonized the land although the genomes have lost some genes during this evolutionary period
intergenic regions have expanded in angiosperm mitochondria
the evolutionary trend of the angiosperm mitochondrial genome is counter to that of the mammalian mitochondrial genome
another characteristic of angiosperm mitochondrial genomes is the high degree of variation in terms of the genomic organization
frequent rearrangements are still ongoing in almost every angiosperm lineage

the complete nucleotide sequences of 15 mitochondrial genomes from seven angiosperm species have been made available (Table 1)
the sizes of the master chromosomes of the sequenced mitochondrial genomes range from 221,853 (rapeseed) to 739,719 bp (maize CMS-C; Fig. 1)
the number of mitochondrial genes in angiosperms is 50–60 (not considering copy number)
some of the mitochondrial genes in angiosperms are interrupted by introns
in each of the sequenced genomes, the total number of the introns is 20–24, constituting 4–13% of the genome
all the introns in the sequenced mitochondrial genomes are classified as group II type
a horizontally transferred group I intron has also been documented
one of the characteristics of angiosperm mitochondrial introns is the presence of trans splicing, particularly for genes encoding Complex I subunits

RNA editing is a necessary step for angiosperm mitochondrial gene expression, which converts cytidine residues to uridine and, rarely, uridine to cytidine
most of the RNA editing sites are found in the protein-coding genes, and a few are in tRNAs, untranslated regions, and introns (Shikanai, 2006)
the numbers of editing sites have been determined for some of the sequenced angiosperm species
a total of 357 in sugar beet
491 in rice

the gene-coding regions constitute only 7–17% of the mitochondrial genome in the angiosperms sequenced to date
the remaining regions (intergenic regions) were examined to discover how the genome sizes had expanded in angiosperms
no conclusive mechanism has been proposed yet
some intergenic regions contain chloroplast DNA sequences (1.6–6.2% of the genome) and nuclear DNA sequences (0.1–13.4% of the genome)

the arrangement of mitochondrial genes can vary, even within a single angiosperm species
angiosperm mitochondrial genomes have undergone frequent genome rearrangement
rearrangement points are unevenly distributed in the genome
the reason for this is unclear
most of the rearrangements do not affect gene expression
they just produce a polymorphism between the genomes
in rare cases they are critical for mitochondrial gene expression

in angiosperms, the most commonly seen class of mitochondrial mutant is cytoplasmic male sterility
efforts to identify the responsible genes in mitochondrial genomes resulted in the discovery of aberrant open reading frames, consisting of fragments of mitochondrial genes and/or unknown sequences, usually with a chimeric structure
in a few cases, aberrant ORFs are translated into unique polypeptides associated with mutant phenotypes
aberrant ORFs can also exist within gene-coding regions such as the 5' leader of the atp6 gene

a number of aberrant ORFs, each producing unique polypeptides, have been reported to be associated with CMS
the aberrant ORFs have little in common, even when their phenotype is very similar
the CMS-associated aberrant ORFs have arisen independently during the course of angiosperm evolution

accumulation of CMS-associated proteins may affect all the tissues but that there is a mechanism of compensation in non-anther tissues

CMS-associated aberrant ORFs often neighbor, and are co-transcribed together with genes for ATPase subunits
from an evolutionary viewpoint, comparative analyses of sequenced mitochondrial genomes revealed that flanking regions of the genes for ATPase subunits are less well conserved compared to those of other genes
suggesting that mutations can accumulate more readily in these regions