THE SUGAR-BEET MITOCHONDRIAL GENE FOR THE ATPASE ALPHA-SUBUNIT - SEQUENCE, TRANSCRIPTION AND REARRANGEMENTS IN CYTOPLASMIC MALE-STERILE PLANTS

We have characterized the mitochondrial atpA (the alpha subunit of F1-ATPase) gene from male-fertile cytoplasm (cv TK81-0) of sugar beet. The gene is 1518-bp long and encodes a polypeptide of 506 amino acids. The atpA mRNA sequence is modified by three C-to-U RNA editing events, all of which alter the encoded protein sequences. In order to examine the genome organization of the atpA locus in cytoplasmic male-sterile (CMS) sugar beet, atpA-containing clones were isolated from Owen CMS (TK81-MS) and a different source of CMS [I-12CMS(2)] cytoplasm respectively. The sequences of the atpA coding region from TK81-MS and I-12CMS(2) are identical to each other and to the corresponding TK81-0 sequence. However, the TK81-0 and TK81-MS loci diverge completely 47 bp upstream of the initiation codon, resulting in different 5' transcript termini for the two genes. On the other hand, the point of divergence between the TK81-0 and I-12CMS(2) atpA genes was found to occur after 393 bp 3' to the TAA stop codon. Our results also show the 3'-flanking sequences of I-12CMS(2) atpA to be present elsewhere in the mitochondrial genomes of TK81-0, TK81-MS and I-12CMS(2), suggesting the possible involvement of these repeated DNA elements in the sequence rearrangements.