A new type of mitochondrial mutation in Paramecium

Inter-species sequence diversity in the replication initiation region of Paramecium mitochondrial DNA

Mitochondrial DNA from Paramecium aurelia is a linear molecule. Replication is initiated at a unique cross-linked molecular terminus. During replication dimer length molecules, consisting of two head-to-head monomers, are generated. We have cloned the head-to-head dimer initiation region from five different species and several stocks (or races) within species and determined its DNA sequence. For all species, this dimer initiation region consists of a central non-palindromic sequence containing almost exclusively A and T, arranged in an array of direct tandem repeats. In an intra-species comparison, the sequences of the repeat units are relatively homogeneous; inter-species comparisons, however, show diversity except for a conserved "Goldberg-Hogness box", T-A-T-A-A-A-T-A. The size of a repeat unit and the number of repeats within a molecule can vary over a wide range, even in an intra-species comparison. Because of these wide inter-species variations observed, it is likely that the function of this region imposes few constraints on the sequence other than its high A + T content and possibly a Goldberg-Hogness box. The array of direct tandem repeats may have arisen from unequal recombination or crossover within this region. Adjacent to the non-palindromic region is a transcribed sequence which is highly conserved for all species and presumably represents a gene coding region.

Genetic interactions in the control of mitochondrial functions in Paramecium. I. Interactions between nuclear genes

The genetic and physiological properties of two nuclear mutants of Paramecium tetraurelia affecting mitochondrial properties, and first screened as resistant to tetrazolium (TTC) are described. The mutant TTC64-1R is strongly deficient in cytochrome c and the mutant TTC66pR is partially deficient in cytochrome aa3; both mutants display cyanide insensitive respiration in exponential growth phase. In the double mutant TTC64-1R -- TTC66pR/TTC64-1R -- TTC66pR the deficiency in cytochrome aa3 due to the TTC66pR mutation is suppressed. The mutation TTC64-1R does not suppress cytochrome aa3 deficiencies due to mitochondrial mutations, but does interact with another nuclear mutation, cl1, (compatible only with mitochondria deficient in cytochrome oxidase) in such a way that the double mutant TTC64-1R -- cl1/TTC64-1R -- cl1 displays a normal amount of cytochrome aa3. The possible mechanisms and physiological significance of these suppressive effects are discussed.

Genetic interactions in the control of mitochondrial function in Paramecium. II. Interactions between nuclear and mitochondrial genomes

In an attempt to understand the genetic interactions between nuclear and mitochondrial genomes leading to mitochondrial biogenesis, different combinations of known nuclear and mitochondrial mutations have been constructed by microinjection. Eleven different tetrazolium resistant mutant strains, many clearly affecting mitochondrial function, were injected with mitochondria from four different erythromycin resistant mitochondrial mutants. Cases were found in which mutant mitochondria were unable to replicate in tetrazolium resistant mutants. The successful mitochondrial transfers were characterized for growth rate, temperature and cold sensitivity. Several selected combinations were characterised also for cytochrome spectra and cyanide resistance. Many different phenotypes were produced by the interaction of the different nuclear and mitochondrial mutations. These ranged from a positive interaction in which mutant mitochondria were selected by a nuclear mutant in preference to wild-type, through apparent absence of interaction, to negative interaction in which the mitochondrial-nuclear combination was temperature sensitive even though both 'parents' were thermoresistant. The possible molecular basis of these interactions is discussed.

The respiratory chain of Paramecium tetraurelia in wild type and the mutant Cl1. I. Spectral properties and redox potentials

1. Purified mitochondria have been prepared from wild type Paramecium tetraurelia and from the mutant Cl1 which lacks cytochrome aa3. Both mitochondrial preparations are characterized by cyanide insensitivity. Their spectral properties and their redox potentials have been studied. 2. Difference spectra (dithionite reduced minus oxidized) of mitochondria from wild type P. tetraurelia at 77 K revealed the alpha peaks of b-type cytochrome (s) at 553 and 557 nm, of c-type cytochrome at 549 nm and a-type cytochrome at 608 nm. Two alpha peaks at 549 and 545 nm could be distinguished in the isolated cytochrome c at 77 K. After cytochrome c extraction from wild type mitochondria, a new peak at 551 nm was unmasked, probably belonging to cytochdrome c1. The a-type cytochrome was characterized by a split Soret band with maxima at 441 and 450 nm. The mitochondria of the mutant Cl1 in exponential phase of growth differed from the wild type mitochondria in that cytochrome aa3 was absent while twice the quantity of cytochrome b was present. In stationary phase, mitochondria of the mutant were characterized by a new absorption peak at 590 nm. 3. Cytochrome aa3 was present at a concentration of 0.3 nmol/mg protein in wild type mitochondria and ubiquinone at a concentration of 8 nmol/mg protein both in mitochondria of the wild type and the mutant Cl1. Cytochrome aa3 was more susceptible to heat than cytochromes b and c,c1.

The respiratory chain of Paramecium tetraurelia in wild type and the mutant Cl1. II. Cyanide-insensitive respiration. Function and regulation

1. The cyanide-insensitive respiration in Paramecium tetraurelia was found to be located in mitochondria. 2. Sensitivity of the mitochondrial respiration to cyanide depended on growth conditions. Under standard conditions of growth, 15--20% of respiration was insensitive to 1 mM cyanide. Full resistance to 1 mM cyanide was observed by growing cells in the presence of erythromycin (100--400 microgram/ml) 0.2 mM cyanide. The mitochondrial respiration of the mutant Cl1 harvested during the exponential phase of growth was largely insensitive to cyanide (more than 80%). 3. Pyruvate was oxidized at the same rate by wild type mitochondria and mitochondria of the mutant Cl1. In contrast, succinate oxidation was 2--3 times faster in mitochondria of the mutant Cl1 than in wild type mitochondria. 4. The cyanide-insensitive respiration was inhibited by 1 mM salicylhydroxamic acid to nearly 100%. Other efficient respiratory inhibitors included amytal and heptylhydroxyquinoline. Antimycin was not inhibitory even at concentrations as high as 5 microgram/mg protein, a finding consistent with the lack of antimycin binding sites.

Selection and characterization of nuclear mutations affecting mitochondria in Paramecium

A screening method, based upon resistance to a tetrazolium salt (TTC), is described which permitted the isolation in Paramecium of 28 mutants resistant to TTC. These mutants displayed various defects in mitochondrial functions (cytochromic content cyanide insensitive respiration). Some mutations seemed to affect directly the respiration chain while others seemed to cause indirect modifications, possibly altering mitochondrial protein synthesis. Genetic analysis of four mutants showed in all that the resistance to TTC was of nuclear origin.