Induction of DNA polymerase activity in irradiated Tetrahymena cells

Purification and characterization of an inducible mitochondrial DNA polymerase from Tetrahymena thermophila

Treatment of the eukaryotic organism Tetrahymena with various types of DNA-damaging agents has been reported to cause a 35-fold induction of a mitochondrial DNA polymerase. We here report that the enzyme can be induced in large-scale cultures by exposure of the cells to thymine starvation and/or intercalating agents. The induced DNA polymerase has been purified to near homogeneity, with a specific activity of approx. 300,000 units/mg protein. The relative molecular mass of the active form of the enzyme is approx. 100,000, as determined by glycerol gradient sedimentation. The subunit structure has been analysed by SDS polyacrylamide gel electrophoresis of the highly purified preparation and by immunoprecipitation with a monoclonal antibody directed to the DNA polymerase. A polypeptide of Mr 47,000 has been observed to be a subunit of the enzyme. This corresponds to the size of the subunits suggested for mitochondrial DNA polymerase from chicken embryos and mouse myeloma cells.

Replicative intermediates of ram sperm mitochondrial DNA

Replication of mtDNA from ejaculated ram spermatozoa was demonstrated by electron microscopy. Twelve percent of mtDNA circular molecules isolated either from ram sperm cells or from rat liver mitochondria were double-forked. These replicative intermediates appeared in four distinct steps of replication: 3.9%, 21.0%, 27.8%, and 44% of the circular contour length. The biological significance of sperm mitochondrial DNA replication is discussed.

Accumulation of replicative DNA intermediates in Tetrahymena after excision-repairable damage to DNA

Inhibition of DNA synthesis in the eukaryotic organism Tetrahymena pyriformis by thymine starvation results in the formation and accumulation of a distinct class of DNA during the first hour after re-initiation of the synthesis. This DNA, when synthesized in the presence of 5-bromodeoxyuridine (BrdUrd), bands as a distinct peak at an intermediate density of (see article) in a neutral CsCl buoyant density gradient. Following short pulses of BrdUrd, the DNA of intermediate density accounts for up to 90% of the newly synthesized DNA and binding assays on nitrocellulose filters show the presence of single-stranded regions. Alkaline buoyant density and alkaline velocity gradients clearly demonstrate the presence of non-covalently linked newly synthesized fragments with an average length shorter than that of the parental strands. Pulse experiments show that the DNA of intermediate density is progressively converted to fully hybrid (light - heavy) DNA molecules. It is therefore suggested that in response to damage caused by thymine starvation, a replicative DNA intermediate accumulates at the growing point. A similar phenomenon has also been observed after irridation with ultraviolet light. The effect of the protein synthesis inhibitor cycloheximide on the formation on the intermediate and the fully hybrid material is also described.

The absence of a pyrimidine dimer repair mechanism in mammalian mitochondria

We have investigated whether mammalian cells can repair pyrimidine dimers in their mitochondrial DNA which have been induced by ultraviolet light. The assay system is based upon the ability of the phage T4 UV endonuclease to nick covalently closed circular mitochondrial DNA that contain pyrimidine dimers. Our results show that dimers are not removed from the mitochondrial DNA of mouse L cells or human KB and HeLa cells. There is also no evidence for photoreactivation of mitochondrial DNA. Analyses of ethidium bromide-cesium chloride equilibrium density gradients of mitochondrial DNA isotopically labeled before and after exposure to ultraviolet light show that the total amount of DNA replication is depressed after exposure. In addition, an increase in the frequency of molecules banding at a position expected for intermediate replicating forms and open circular daughter molecules suggests that the rate of replication is slower (or arrested) in molecules with pyrimidine dimers. The absence of a significant amount of mixing of label incorporated before and after ultraviolet-irradiation is evidence against the occurrence of a large amount of genetic exchange between mitochondrial DNA molecules under these conditions.