Effect of starvation for glucose during reversion of a long term adapting yeast.

The mass reversion phenomenon in long term adapting yeasts was discovered by Spiegelman et al. (1950). The essential feature is that, whereas a galactose-grown population of cells of the gs genotype forms entirely positive colonies on galactose indicator plates, subculture of such a population on medium not containing galactose results in a sudden, mass change of the culture to a type that forms negative colonies. This change was termed reversion because the galactose-grown culture used was descended from a glucose-grown culture which likewise produced mostly negative colonies on galactose plates. Later (Spiegelman et al., 1953), evidence was presented suggesting the following model for reversion: (1) Reversion involves the dilution with growth of autocatalytic particles necessary for the formation of some enzyme(s) required for galactose fermentation. (2) Any cell which contains one or more such particles at the time of plating will give rise to a positive colony. (3) During growth in a galactose free medium, there is neither multiplication nor destruction of particles. The reversion generation of a culture was defined by Campbell and Spiegelman (1956) as the number of generations of growth in galactosefree medium which that culture must undergo before 50 per cent of its cells produce negative colonies. According to the above model, the reversion generation is proportional to the logarithm of the average number of particles per cell. This constitutes the sole method for estimating this number. It has been used (Campbell and Spiegelman, 1956) to investigate the growth kinetics of the postulated particles. The subject of the present communication is 3