Studies on the mitochondrial DNA of Tetrahymena
The mitochondrial DNA from the ciliate protozoan Tetrahymena furgasoni str. W/ATCC. was isolated and mapped using six restriction enzymes. The linear molecule was seen to be approximately 35 Md. in size, slightly larger than the mtDNA seen in other Yetrahymena strains. The T. furgasoni mtDNA molecule also showed a heterogeneity in the length of its terminal regions, a characteristic of Tetrahymena mtDNA.The position of the mitochondrial rRNA genes were mapped on the molecule by hybridization studies using isolated mitochondrial rRNA's. The T. furgasoni mtDNA molecule possesses two large rRNA genes, one at each end of the molecule in sub-terminal locations, and a single small rRNA gene. A third, incomplete large rRNA gene was also found. The presence of this extra, incomplete rRNA gene may indicate why T. furgasoni mtDNA is slightly larger than the mtDNA from other Tetrahymena strains.The terminal Hind III restriction fragment from the mtDNA of T. pyriformis was cloned using the vector pJB 8. Three copies of the fragment were cloned. These three recombinant molecules were different in size, a difference which was associated with the size of the original terminus of the mtDNA molecule. DNA sequencing studies showed the difference in length to be associated with a variation in the number of copies of a 31 bp repeat sequence present at the original terminus of the mtDNA molecule.The significance of this mtDNA terminal structure is discussed with respect to the completion of replication of the linear mtDNA molecule, and to the generation of the terminal length heterogeneity of the linear mtDNA molecule. The structural characteristics of the Tetrahymena mtDNA terminus are compared with the structures seen at the termini of other linear genetic elements from a variety of sources.