Organization and post-transcriptional modification of mitochondrial genes in higher plants

Recent topics regarding higher plant mitochondria are briefly reviewed, such as gene organization, the genes associated with mutants, and the maturation of transcripts. The estimated genome size of higher plant mitochondrial DNA varies from about 200 kb to 2,500 kb. Most of the mitochondrial genomes are organized as complex multiple circular molecules, generated from recombinations between repeated sequences. Several protein-encoding genes unique to plant mitochondria are identified but gene contents are not identical among the plant species. Gene transfer of cytochrome oxidase subunit II, generally mitochondrially encoded gene, is found to move from mitochondrion to nucleus via RNA molecules and the event is limited in a single genus of legume. Genes associated with mitochondrial mutants, CMS-T and NCS3, are isolated and characterized in detail. Plant mitochondria have two types of notable post-transcriptional RNA modifications. One is RNA editing that alters a cytidine residue in DNA to a uridine residue in RNA, and results in an evolutionally more conservative polypeptide. The other istrans-splicing of transcripts of NADH dehydrogenase subunit 1 and 5 genes, that fuses the physically scattered mRNAs and makes a mature mRNA.