Low molecular weight circular and linear DNA in mitochondria from normal and male-sterile Zea mays cytoplasm

Abstract

Maternally inherited cytoplasmic variation in plants is well documented¹. One consequence of such variation in maize (Zea mays L.) is male sterility, sources of which have been classified into three groups (T, S and C) depending on nuclear gene fertility restoration^2–4. These groups and the normal (N) male-fertile cytoplasm can be differentiated by restriction endonuclease analysis of mitochondrial DNA (mtDNA)^5,6, although differences have also been found within the N and C groups^7,8, and mitochondrial translation products^9,10. Encouraged by the finding that mitochondria of S-cytoplasmic types contain discrete small DNA species¹¹, we have looked for low molecular weight DNAs in mitochondria from plants with N, T, C and S cytoplasms. We report here that all four cytoplasms contain supercoiled circular DNA molecules of approximately 1,940 base pairs. This is the first small supercoiled circular DNA to be found in higher plants. Its structure suggests it may be an autonomously replicated plasmid. N, C and S cytoplasms also contain a DNA species of approximately 2,350 base pairs which is not present in T cytoplasms. C cytoplasms contain two additional circular DNA species of 1,570 and 1,420 base pairs.