Mutation of the non-Mendelian suppressor, Psi, in yeast by hypertonic media.

The Psi(+) extrachromosomal determinant in the yeast Saccharomyces cerevisiae suppresses certain UAA markers and increases the efficiency of suppression of UAA suppressors and certain frameshift suppressors. Although the exact nature of Psi(+) determinant is unknown, it is believed to be a self-replicating cytoplasmic factor affecting some component of the translational machinery. In this report we describe growth conditions for efficient mutation or elimination of the Psi(+) determinant. Incubation of Psi(+) cultures in hypertonic nutrient medium resulted in rapid conversion to a culture containing predominantly Psi(-) cells during the growth cycle. The kinetics of Psi(+) to Psi(-) conversion established that the occurrence of Psi(-) cells was due to induction and not to selection of pre-existing Psi(-) cells. The results suggest that the replication of the Psi(+) determinant is sensitive to hypertonic conditions.