SUMMARY: Spontaneous and u.v.-induced revertants were found to be unstable in serial subculture. Instability was manifested by () replacement of the original revertant by revertants with slower growth rates on minimal medium, and () the appearance of auxotrophic organisms which overgrew in some cases but not others. In one case, the instability was found to be more pronounced when the organisms were maintained in the logarithmic phase of growth. This evidence, along with the previous radiological evidence, was interpreted according to the following hypothesis. The auxotrophic organism has an episomic element located at the suppressor locus. The damage inflicted by radiation consists in the detachment of this episome. The prototrophic cell may again become auxotrophic by reattachment.


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