The yeast [PSI+] element represents an aggregated form of release factor Sup35p and is inherited by a prion mechanism. A "species barrier" prevents crosstransmission of the [PSI+] state between heterotypic Sup35p "prions." Kluyveromyces lactis and Yarrowia lipolytica Sup35 proteins, however, show interspecies [PSI+] transmissibility and susceptibility and a high spontaneous propagation rate. Cross-seeding was visualized by coaggregation of differential fluorescence probes fused to heterotypic Sup35 proteins. This coaggregation state, referred to as a "quasi-prion" state, can be stably maintained as a heritable [PSI+] element composed of heterologous Sup35 proteins. K. lactis Sup35p was capable of forming [PSI+] elements not only in S. cerevisiae but in K. lactis. These two Sup35 proteins contain unique multiple imperfect oligopeptide repeats responsible for crosstransmission and high spontaneous propagation of novel [PSI+] elements.
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