Characterization of circular mitochondrial plasmids in three Pythium species

Four circular plasmids, with a monomer size ranging from 3.2 to 4.94 kb, have been identified in isolates of P. aphanidermatum (two different plasmids), P. torulosum, and an unidentified echinulate isolate. The mitochondrial location has been confirmed for three of the plasmids. Each fungal isolate contained a single plasmid, present in both monomeric and oligomeric forms; plasmid monomers were present as open circles and as supercoiled forms. Restriction maps of the plasmids were dissimilar. Hybridization studies using cloned plasmids revealed no DNA sequence similarity among the different plasmids or between the plasmids and the nuclear or mitochondrial genome of the isolates from which they were recovered. Hybridization of labeled plasmid DNA to Northern transfers of mitochondrial RNA for two isolates indicate that what appears to be the predominant RNA transcript is unit length in size. For three isolates, the plasmid was retained following subculturing and was present in all asexual and sexual single-spore progeny evaluated. For one isolate of P. aphanidermatum the plasmid was unstable and was lost during subculturing.