Abstract
The anomalous reaction of a respiratory-deficient strain of Saccharomyces cerevisiae to u.v. irradiation was investigated. Individual lines of this strain became moribund after reproducing for a variable number of cell generations on a medium normally considered optimal. The progression toward extinction could be halted by exposure of cells to u.v. light. Six influences were found which mimicked the effect of irradiation. These were: (1) a reduction of nutrients in the culture medium, (2) mutation, (3) substitution of galactose for dextrose in the culture medium, (4) an increased temperature of incubation, (5) a decreased temperature of incubation, and (6) a spindle poison, mercaptoethanol. In each case the increase in life expectancy of the strain was accompanied by a reduced rate of cell division. By analogy, the beneficial effect of irradiation was attributed to the inhibition of mitosis. It was considered that this inhibition permitted the accumulation of an essential substance which would otherwise be depleted and lead to cell death. The nature of this substance remains to be determined, but results indicated that its production is relatively unaffected by irradiation.
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