Organization of sugar beet mitochondrial genome

To date, sugar beet (Beta vulgaris) and Arabidopsis thaliana are the only angiosperms for which the complete nucleotide sequence of the mitochondrial genome is available. The sugar beet mitochondrial genome contains 29 protein, 5 rRNA and 25 tRNA genes, most of which are also shared by the mitochondrial genome of Arabidopsis. However, four of these genes are missing in Arabidopsis mitochondria. In addition, four genes found in Arabidopsis are entirely absent in sugar beet, or present only in severely truncated form. Although the overall sizes of the genomes are similar to each other, the sequences common to both species constitute only about 21 % of the genome, and the remaining part consists of unique sequences. We identified a novel tRNACys gene (trnC2-GCA) in sugar beet mitochondria which shows no sequence homology with any tRNACys genes reported so far in higher plants. Sequences homologous to trnC2-GCA were detected in other Caryophyllalean plants. Introduction Despite the fact that mitochondria have essentially the same metabolic function in all eukaryotes which contain them, higher plant mtDNAs exhibit a number of unique structural features (e.g. large size, rapid evolutionary rearrangements and chloroplast DNA insertions) and specific modes of expression (e.g. cisand trans-splicing, RNA editing and universal genetic code usage) (Schuster and Brennicke, 1994). Moreover, they encode significantly more genes than do their fungal and animal counterparts. Ribosomal protein genes as well as the genes involved in cytochrome c biogenesis comprise a significant fraction of such additional genes identified so far in plant mitochondria (Schuster and Brennicke, 1994). It is also interesting to note that the study of different plant lineages has demonstrated the recent evolutionary transfer of certain mitochondrial genes to the nucleus, and therefore the gene content of mitochondria may vary in different species (Schuster and Brennicke, 1994). The determination of and comparison between the complete mtDNA sequences of two angiosperms would thus allow us to examine the entire coding potential of plant mitochondrial genomes and to ascertain whether or not the plant mitochondrial gene content and organization have been conserved during flowering plant evolution. In Endocytobiosis Cell Res. (2004) 15, 87-95