Degree Name

Master of Science (Hons.)


Department of Biology


An increased proportion of petite mutants was observed when mid-exponential cultures of Saccharomyces cerevisiae were transferred to a salt broth containing 10% NaCl as compared to a low spontaneous proportion that arose 'before transfer' (normal culture) to the salt broth. The proportion of mutants arising from salt exposure is a characteristic of individual yeast strains. This increase in proportion of mutants appeared to be due to the mutagenic action of the salt stress, rather then an enhanced survival of the pre-existing petite strains before salt stress. Petite mutants were not inherentiy more resistant to salt stress than the parent strains. Changes in viability and proportion of petite mutants caused by a sudden transfer of mid-exponential cultures to salt broth were affected by: age of culture, temperature, and preconditioning (either in 2% NaCl or 45 °C). Stationary phase cultures were resistant to transfer to salt broth as compared to mid-exponential cultures. Thus, the proportion of mutants was always low from stationary cultures, reflecting the spontaneous level before stress. The effects of temperature on salt stress-induced petite mutagenesis were also studied. High and low temperatures were used such as 20°C and 52°C. A temperature sensitive mutant, strain 182-6-3 (cdc 24), was used in experiments at 20°C and the wild type, Y41, and a previously considered to be trehalose negative mutant, 211-244-1A {glc 1), were used in experiments at 52°C. Results with cdc 24 showed an increase in petite mutagenesis when a mid-exponential culture was transferred to salt broth at 20°C. On the other hand, Y41 and glc 1 showed increases in petite mutagenesis when a mid-exponential culture was transferred to BYM at 52°C. However, when a Y41 mid-exponential culture was transferred to a salt broth (BYM containing 10% NaCl) at 52°C, the mutation rate was decreased and the apparent viability was increased compared to viability achieved from cells in BYM. This indicates that high NaCl concentration and high temperature oppose one another, thereby reducing the lethal effects on viability, and decreasing the mutation rate. Preconditioning of mid-exponential cultures either in 2% NaCl or 45°C also affects both the effect of transfer to salt broth (at 30°C) or BYM at 52°C. For example. preincubation in 2% (m/v) NaCl suppressed the lethal effect of a sudden transfer to salt broth (10% m/v, NaCl at 30°C). Different effects were noted for the three yeast strains after such treatment. There was little protection for glc 1. The 2% NaCl pretreatment may be effective because it causes glycerol accumulation (Mackenzie et al. 1986) but the results obtained with glc 1 suggest that trehalose might also be involved in the stress protection. Pretreatment at 45°C protects against lethal effect of salt stress, but does not seem to suppress mutation. Both 2% NaCl and 45°C pre-conditioning suppress petite mutagenesis induced at 52°C. Trehalose accumulated to a maximum of 18.7 and 4.1 mg. g"^(dry mass of yeast) respectively within strains Y41 and glc 1. Trehalose may be a protector against heat and salt stress-induced petite mutagenesis.