Mitochondrial DNA: Advances, Problems, and Goals

Mitochondrial genetics, circular DNA and the mechanism of the petite mutation in yeast

We propose a general hypothesis involving properties of circular DNA which can explain such phenomena as thepetitemutation, suppressiveness, and the polarity observed in mitochondrial recombination in the yeastSaccharomyces cerevisiae. This hypothesis involves excision and insertion events between circular DNA molecules as well as structural rearrangements in the DNA generated by these events. The special properties of circular DNA have been considered in analysing recombination, and a number of results are obtained which are not intuitively apparent.

This hypothesis can be applied to any situation involving circular DNA such as bacterial plasmids and cytoplasmic circular DNAs, where the opportunity exists for recombination and rearrangement events.

Temperature-dependent formation of dimers and oligomers of mitochondrial DNA in cells transformed by a thermosensitive mutant of Rous sarcoma virus

In chick-embryo fibroblasts infected by T5, a temperature-sensitive mutant of Schmidt-Ruppin Rous sarcoma virus, the level of catenated dimeric and oligomeric mitochondrial DNA is temperature-dependent and correlates with the phenotypic manifestation of transformation. At the permissive temperature (36 degrees ), where transformation is expressed, the 2- to 3-fold elevated level characteristic of cells transformed by the wild-type Rous sarcoma virus was observed, but at the nonpermissive temperature (41 degrees ), at which cells appear normal and behave normally, the oligomer level was characteristic of uninfected cells. Temperature shifts from 41 to 36 degrees and vice versa resulted in phenotypic reversion and reversals of the respective levels of multiple-length DNA. Cellular growth rates were not altered. Treatment of control cells with cycloheximide resulted in a 5-fold increase of oligomeric DNA, whereas exposure to 9-beta-D-arabinofuranosyladenine had little effect. With both inhibitors, nuclear but not mitochondrial DNA synthesis was inhibited. The possible relation of formation of oligomeric mitochondrial DNA to alterations in the mitochondrial membranes of transformed cells and to inhibition of protein synthesis in the cytoplasm is discussed.

Integrated state of oncornavirus DNA in normal chicken cells and in cells transformed by avian myeloblastosis virus

The covalent linkage of oncornavirus-specific DNA to chicken DNA was investigated in normal chicken embryo fibroblasts (CEF) and in virus-producing leukemic cells transformed by avian myeloblastosis virus (AMV). The virus-specific sequences present in cellular DNA fractionated by different methods were detected by DNA-RNA hybridization by using 70S AMV RNA as a probe. In CEF and in leukemic cells, the viral DNA appeared to be present only in the nucleus. After cesium chloride-ethidium bromide density equilibrium sedimentation, the viral DNA was present as linear, double-stranded molecules not separable from linear chicken DNA. After extraction by the Hirt procedure, the viral DNA precipitated with the high-molecular-weight DNA. After alkaline sucrose velocity sedimentation, the viral DNA cosedimented with the high-molecular-weight cellular DNA. The results indicate that in both types of cells studied, the oncornavirus-specific DNA sequences were linked by alkali stable bonds to nuclear cellular DNA of high molecular weight and did not appear to be present in free form of any size.

[The activity of phosphoglucose-isomerase and phosphoglucomutase during morphogenesis of nucleate and anucleate cells of Acetabularia]

The activity of two enzymes of the UDP-glucose and UDP-galactose biosynthetic pathway-phosphoglucose-isomerase and phosphoglucomutase-have been followed during morphogenesis of the unicellular green alga Acetabularia mediterranea. We have found that the increase in enzyme activities during morphogenesis represents de novo synthesis. The kinetic constants (K m, K i) of phosphoglucose-isomerase are similar to those found for the enzyme from rabbit muscle; mannose-6-phosphate was found to be a competitive inhibitor.The increase in activity of phosphoglucose-isomerase parallels the linear increase in total protein, neither showing changes with cap formation. In contrast, the activity of phosphoglucomutase is enhanced markedly when cap formation starts.Whether the nucleus is present or absent does not affect the above mentioned properties of the two enzymes. From this fact we conclude that the enzyme syntheses are regulated by the cytoplasm, probably at the level of translation. Contributions of DNA from chloroplasts and mitochondria can be excluded because in anucleate cells, which have an already established gradient of m-RNA, actinomycin D (10 μg/ml) has no effect on the synthesis of these enzymes.There are also differences in the spatial distribution of the two enzymes. We were able to show that both enzymes are synthesized unequally in different regions of the cells.