Analysis of [SWI+ ] formation and propagation events.

The budding yeast, Saccharomyces cerevisiae, harbors several prions that are transmitted as altered, heritable protein conformations. [SWI+ ] is one such prion whose determinant is Swi1, a subunit of the evolutionarily conserved chromatin-remodeling complex SWI/SNF. Despite the importance of Swi1, the molecular events that lead to [SWI+ ] prionogenesis remain poorly understood. In this study, we have constructed floccullin-promoter-based URA3 reporters for [SWI+ ] identification. Using these reporters, we show that the spontaneous formation frequency of [SWI+ ] is significantly higher than that of [PSI+ ] (prion form of Sup35). We also show that preexisting [PSI+ ] or [PIN+ ] (prion form of Rnq1), or overproduction of Swi1 prion-domain (PrD) can considerably promote Swi1 prionogenesis. Moreover, our data suggest a strain-specific effect of overproduction of Sse1 - a nucleotide exchange factor of the molecular chaperone Hsp70, and its interaction with another molecular chaperone Hsp104 on [SWI+ ] maintenance. Additionally, we show that Swi1 aggregates are initially ring/ribbon-like then become dot-like in mature [SWI+ ] cells. In the presence of [PSI+ ] or [PIN+ ], Swi1 ring/ribbon-like aggregates predominantly colocalize with the Sup35 or Rnq1 aggregates; without a preexisting prion, however, such colocalizations are rarely seen during Swi1-PrD overproduction-promoted Swi1 prionogenesis. We have thus demonstrated a complex interacting mechanism of yeast prionogenesis.