GENOMIC ORGANIZATION AND SEQUENCE-ANALYSIS OF THE CYTOCHROME-OXIDASE SUBUNIT-II GENE FROM NORMAL AND MALE-STERILE MITOCHONDRIA IN SUGAR-BEET

We have cloned and sequenced the cytochrome oxidase subunit II (coxII) gene from both normal and cytoplasmic male-sterile (CMS) sugar beet. The normal coxII (designated NcoxII) locus was found to be located 1491 bp upstream from the gene for cytochrome oxidase subunit I (coxI) on the same DNA strand and to have a 1463 bp intron which split the coding sequence into two exons (382 and 398 bp). The COXII protein contains 260 amino acid residues. We have also found two copies of the coxII gene (ScoxII-1 and ScoxII-2) to be present in the CMS genome. Out results suggest that the NcoxII gene diverges completely from the ScoxII-1 and ScoxII-2 genes 50 bp 5' to the ATG start codon. In addition, the ScoxII-1 and ScoxII-2 sequences could be readily discriminated from each other by the 3' end and the immediately adjacent flanking sequences of the gene: the 3' divergence results in a 101 codon extension of the ScoxII-2 ORF. Northern blot analysis demonstrates that the coxII gene exhibits altered transcript patterns in CMS compared with normal sugar beet. Different genomic arrangements of the coxII gene are considered to be the result of extensive intra- and inter-molecular recombination events involving the repeated DNA elements in the mitochondrial genome.