Molecular cloning and physical characterization of a Brassica linear mitochondrial plasmid

A linear mitochondrial plasmid reported to be associated with cytoplasmic male sterility in the genus Brassica was analyzed. A protein was found to be associated with the 5' ends of the plasmid. The entire plasmid was cloned by the homopolymer tailing technique via free hydroxyl groups present at its 3' ends. DNA sequence analysis of the cloned plasmid revealed a perfect terminal inverted repeat of 325 base pairs. Southern hybridization and restriction enzyme mapping analysis confirmed colinearity of the native plasmid and the clone, which showed significant homology with organelle DNA but not with nuclear DNA. Under high-stringency hybridization conditions, an internal 4.6 kb fragment of the 11.5 kb plasmid hybridized to the main mitochondrial genome in several species. Although the hybridization signal was weaker, the chloroplast genome also showed homology to the mitochondrial plasmid. The plasmid was undetectable at a molar ratio of less than 1/10 000 of the main mitochondrial genome in some lines of Brassica and Raphanus that contain the Ogura male sterile cytoplasm (cms). The absence of the plasmid in these sterile lines demonstrates that the plasmid is not required for the expression and maternal inheritance of male sterility.