Circular plasmid DNAs from mitochondria of Sorghum bicolor

Agarose gel electrophoresis of mitochondrial DNA (mtDNA) from the IS1112C male-sterile cytoplasm of Sorghum bicolor (S. bicolor) revealed plasmid-like DNAs additional to the linear N1 and N2 molecules. Mitochondrial plasmids were separated from the principal mitochondrial genome and used in the construction of molecular clones. Clones with EcoRI inserts of 1.7 and 2.3 kb were recovered. Hybridization of these clones to Southern blots of unrestricted and EcoRI-digested IS1112C mitochondrial plasmids indicated the cloned inserts were complete or nearly-complete copies of minicircular DNA molecules. These clones were used to probe Southern blots of mitochondrial genomes from six cytoplasmic male-sterile (CMS) and five male-fertile (MF) lines of S. bicolor, as well as Southern blots of IS1112C chloroplast, kafir chloroplast, IS1112C nuclear, and kafir nuclear genomes. The 2.3 and 1.7 kb plasmids had a very limited distribution among the sorghum entries we examined. We found no evidence for integrated copies of these sequences in any of the mitochondrial, chloroplast, or nuclear genomes probed in this study. However, the 2.3 kb sorghum minicircle did hybridize to the 1.94 kb minicircle from maize mitochondria. Hybridization of the 1.7 and 2.3 kb clones to IS111L2C mitochondrial RNA reveal a transcript of 1.1 kb from the 1.7 kb minicircle and transcripts of 1.9 and 1.4 kb from the 2.3 kb molecule.