The yeast cytoplasmically inherited genetic determinant [PSI ϩ ] is presumed to be a manifestation of the prion-like properties of the Sup35 protein (Sup35p). Here, cell-free conversion of Sup35p from [ psi Ϫ ] cells (Sup35p psiϪ ) to the prion-like [PSI ϩ ]-specific form (Sup35p PSIϩ ) was observed. The conversion reaction could be repeated for several consecutive cycles, thus modeling in vitro continuous [PSI ϩ ] propagation. Size fractionation of lysates of [PSI ϩ ] cells demonstrated that

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... e converting activity was associated solely with Sup35p PSIϩ aggregates, which agrees with the nucleation model for [PSI ϩ ] propagation. Sup35p PSIϩ was purified and showed high conversion activity, thus confirming the prion hypothesis for Sup35p.