Induction of distinct [URE3] yeast prion strains

[URE3] is a non-Mendelian genetic element in Saccharomyces cerevisiae, whic h is caused by a prion-like, autocatalytic conversion of the Ure2 protein ( Ure2p) into an inactive form. The presence of [URE3] allows yeast cells to take up ureidosuccinic acid in the presence of ammonia. This phenotype can be used to select for the prion state. We have developed a novel reporter, in which the ADE2 gene is controlled by the DALS regulatory region, which a llows monitoring of Ure2p function by a colony color phenotype. Using this reporter, we observed induction of different [URE3] prion variants ("strain s") following overexpression of the N-terminal Ure2p prion domain (UPD) or full-length Ure2p. Full-length Ure2p induced two types of [URE3]: type A co rresponds to conventional [URE3], whereas the novel type B variant is chara cterized by relatively high residual Ure2p activity and efficient curing by coexpression of low amounts of a UPD-green fluorescent protein fusion prot ein. Overexpression of UPD induced type B [URE3] but not type A. Both type A and B [URE3] strains, as well as weak and strong isolates of type A, were shown to stably maintain different prion strain characteristics. We sugges t that these strain variants result from different modes of aggregation of similar Ure2p monomers. We also demonstrate a procedure to counterselect ag ainst the [URE3] state.