1)Rumi MIYAMOTO �E 2)Akira UCHIDA

1) Division of Education of the Graduate School, Science Education, Kobe University

2) Division of Natural Environment and Biology, Faculty of Human Development, Kobe University

ABSTRACT

�@�@Reverse mutation rates were examined at leucine locus by using an auxotrophic mutant of S.cerevisiae that was requring uracil, lysine and leucine. The rates were calculated by two methods ; the first method was to calaulate from the proportion of similar and independent small-scale cultures having no revertants and the second was from the average number of revertants in similar and comparatively large-scale cultures.
�@�@The mutation rates at leucine locus in the small-scale cultures all fell into the region of 1�`2�~10-7. However, the rates in the large-scale cultures were drastically changed with the cultural conditions. Growth in minimal medium and starvation for leucine and lysine could induce reverse mutation at leucine locus with high frequencies of 5�`10�~10-2 but growth in complete medium did not. The discrepancy between the reverse mutation rates in the small- and large-scale cultures under different cultural conditions indicates that the reversion at leucine locus would involve, at least, two-step mutational events.

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KEY WORDS: conditional reversion, leucine locus, Saccharomyces cerevisiae
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