Properties of the linear N1 and N2 plasmid-like DNAs from mitochondria of cytoplasmic male-sterile Sorghum bicolor

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Abstract
The linear N1 and N2 plasmid-like DNAs were recovered from mitochondria of the IS1112C line of cytoplasmic male-sterile (CMS) Sorghum bicolor (S. bicolor). Molecular clones containing internal sequences of these plasmids were constructed. These clones were used to probe Southern blots of mitochondrial genomes from six CMS and five male-fertile (MF) lines of S. bicolor, as well as Southern blots of IS1112C chloroplast, IS1112C nuclear and kafir nuclear genomes. We found no evidence for integrated copies of N1 or N2 in any of the mitochondrial, chloroplast or nuclear genomes probed in this study. Our clones did detect an N1-homologous transcript of 3.1 kb and N2-homologous transcripts of 3.9 and 1.4 kb in IS1112C mitochondrial RNA prepared from lines with and without nuclear, fertility-restoring genes. N1 and N2 DNAs were degraded by exonuclease III but were resistant to lambda exonuclease, presumably due to the presence of 5′ terminal proteins. We detected multimeric forms of N1 and N2 in Southern blots of unrestricted, IS1112C mitochondrial DNA (mtDNA). These forms apparently also had associated protein molecules.