URE3] and [PSI] are prions of yeast and evidence for new fungal prions

URE3] and PSI] are two non-Mendelian genetic elements discovered over 25 years ago and never assigned to a nucleic acid replicon. Their genetic properties led us to propose that they are prions, altered self-propagating forms of Ure2p and Sup35p, respectively, that cannot properly carry out the normal functions of these proteins. Ure2p is partially protease-resistant in URE3] strains and Sup35p is aggregated specifically in PSI] strains supporting this idea. Overexpression of Hsp104 cures PSI], as does the absence of this protein, suggesting that the prion change of Sup35p in PSI] strains is aggregation. Strains of PSI], analogous to those described for scrapie, have now been described as well as an in vitro system for PSI] propagation. Recently, two new potential prions have been described, one in yeast and the other in the filamentous fungus, Podospora.