Identification of mitochondrial male sterility gene and molecular dissection of pollen abortion sugarbeet

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We determined the complete nucleotide sequence of the mtDNA from cytoplasmic male sterile sugarbeet (cv.TK81-MS), with the expectation that the data will aid in identifying genes responsible for male sterility. The entire genome comprises 501020 bp. Although the mitochondrial genome of TK81-MS is 130kb larger than the normal fertile mtDNA (TK81-O), there is no difference in the content of identified genes between the two mitochondrial genomes. Most of the additional sequences in TK81-MS mtDNA are composed of sequence duplications and increased intergenic regions of unknown origin. In an attempt to identify the TK81-MS-specific ORF(open reading frame) involved in male sterility, we found that atp6 gene from TK81-MS has a 387-codon presequence which shows no similarity to the presequence of either TK81-O atp6, or any other sequences of TK81-O mtDNA. The TK81-MS atp6 presequence is actually transcribed and translated in the mitochondrion, resulting in the expression of a 35-kDa protein. The variant 35-kDa protein was revealed to be characteristic of the male-sterile mitochondria and to accumulate in flower buds. This study is the first to describe that the atp6 presequence is actually translated in mitochondria and causally related with the male sterility phenotype.