ALBERT

Notes: Summary In the presence of both high glucose concentration and non-fermentable substrate, ethidium bromide initially prevents cell growth. However, after a lag phase (1 day in glucose, 3–12 days under non-fermentative conditions) cells regain capability to grow despite the presence of ethidium bromide. After removal of the drug, this “sensitivity” is lost with 5 generations: Apparently the initial step(s) of the inhibition is (are) reversible. After 8–10 generations' growth in liquid medium in the presence of up to 100 μg/ml ethidium bromide, mitochondrial DNA is neither diminished nor altered in its density. With high-glucose in the culture medium, the content of mitochondrial DNA relative to nuclear DNA is about half that of derepressed cells, both in the presence and absence of the drug. It is ruled out that degradation of the dye is responsible for its ineffectivity on mitochondrial DNA of Schizosaccharomyces pombe. In contrast to Saccharomyces cerevisiae, no petite mutants can be recovered as petite colonies even after prolonged ethidium bromide treatment.

Notes: Summary A sorbose-resistant double mutant sor r A-10/sor r C-17 produces larger colonies in sorbose containing test-medium than the respective single mutants; wildtype colonies remain very small. Resistance of the single mutants was shown to be connected with a decreased rate of sorbose-uptake into their conidia; however, sorbose uptake of the double mutant had not been measured. To check, whether the improved performance of the double mutant on test medium is correlated with a further decrease of sorbose uptake in this strain, studies on the uptake of fructose, sorbose and deoxyglucose by ungerminated conidia of the two single mutants, the double mutant and the wildtype were conducted, using C14-marked sugars, the millipore filter technique, and conidia either untreated or pretreated with 1% sorbose for 4 hours. If sorbose uptake is referred to that of fructose as basis of calculations, as in the earlier studies, the sorbose uptake by cells of the double mutant is smaller than that of both single mutants for conidia not pretreated with sorbose (Fig. 7a). However, for conidia pretreated with sorbose, this correlation does not hold. Rather, cells of the double mutant take up less sorbose than those of the C-mutant, but as much or slightly more than those of the A-mutant (Fig. 7b). If sorbose uptake is referred to that of deoxyglucose for an independent point of reference, cells of the double mutant take up less sorbose than those of the C-mutant, but much more than those of the A-mutant. This holds for untreated and sorbose pretreated cells (Fig. 5 a and b). These data rule out a correlation between colony size and transport defect for at least one of the strains used here, i.e. the C-mutant. The following data suggest a new interpretation: In contrast to the earlier findings with germinated conidia, ungerminated untreated cells of the C-mutant take up much more fructose and sorbose than those of the wildtype (Fig. 3 a and 1a). The uptake of fructose by cells of the C-mutant can not be improved by sorbose pretreatement (Fig. 3 b), but in both wildtype and A-mutant it is increased (Figs. 1b and 2b). Uptake of deoxyglucose was nearly equal for all three strains either untreated or pretreated. Untreated cells of the A-mutant take up as much sorbose as those of the wildtype (Figs. 2 a and 1 a). On pretreatment their sorbose uptake remains nearly constant (Figs. 2b), in contrast to wildtype cells, where it increases drastically and without an increase of fructose uptake by an equivalent amount (Fig. 1b). The new interpretation suggests that gene C is of the regulator type. Mutation of it in the C-strain used here has lead to the simultaneous de-repression of a system for fructose and sorbose uptake. Deoxyglucose uptake is not served by this system. Gene A is a structural gene, harbouring the information for the inducible synthesis of a carrier or permease specifically engaged in sorbose uptake. It is not under the controll of gene C. This interpretation is supported by results on untreated cells of the double mutant. However, fructose uptake of such cells is roughly equal to that of C-mutant cells (Fig. 6a) and sorbose uptake is less (Fig. 5a). Hence, a secondary effect of the A-gene, i.e. on the amount of de-repression of sorbose uptake by mutation in gene C, is indicated.

Notes: Summary In Saccharomyces cerevisiae, mutants were isolated which show high resistance to the aminoglycoside paromomycin. Amino acid incorporation of mitochondria isolated from such mutant strains proved also to be paromomycin resistant. All of them are cross-resistant to the structurally related antibiotic neomycin. Three independent methods revealed the resistance to be extrachromosomally, presumably mitochondrially inherited.