The Process of Spontaneous Extranuclear Mutation in Yeast.

ONVINCING evidence has been provided to show that respiratory mutants C of yeast can arise as a result of extranuclear, as well as nuclear change (EPHRUSSI, HOTTINGUER and TAVLITZKI 1949). The extranuclear mutants are known as " vegetative petites. " At least two classes of them exist (EPHRUSSI et al. 1955). They can be induced by means of specific mutagenic agents, such as the acridine dyes (EPHRUSSI and HOTTINGUER 1950) and radiations (RAUT 1954). When such agents are used, the mutation appears to involve the destruction or inactivation of heritable cytoplasmic particles. Extranuclear variants can also arise spontaneously, and it has been shown that the rates at which these are produced can be affected by the genic constitution of the cells (EPHRUSSI and HOTTINGUER 1951) and by temperature (YCAS 1956). However, the spontaneous process has received less attention than has the induced, and so far, investigations have permitted the underlying mechanism to be explained in general terms only. This report is concerned with the process by which vegetative petites are produced in an unstable strain of yeast. The interpretation of strain behavior is based on the premise that mutation results from the depletion of essential cytoplasmic factors, and the data are applied to an elucidation of the mechanism by which this depletion occurs. It would, however, be difficult to interpret the data in a way that did not involve such particles, and the results of the investigation can be regarded as providing yet further evidence of their existence. Alternative to depletion, interpretations of the mutation process in terms of functional inactivation are possible. However, as the present inquiry is pursued, it will be seen that these become increasingly untenable. The loss of cytoplasmic particles from a cell could be a single and sudden event resulting from the " chance " breakdown of some mechanism controlling the orderly distribution of particles by the mother to the daughter cell. If so, the probability of petite production would be the same for all cells of the strain. Alternatively a mechanism for determining the precise distribution of particles may not exist. In this case the final loss may be simply a result of imprecise assortment. If so, vegetative petites would more commonly occur where a partial reduction in particle number had already taken place. To discriminate between these two possibilities, sudden loss and loss preceded by decline in numbers, a technique was devised by …