Induction of Distinct [URE3] Yeast Prion Strains

Abstract

[URE3] is a non-Mendelian genetic element in Saccharomyces cerevisiae, which 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 DAL5 regulatory region, which allows monitoring of Ure2p function by a colony color phenotype. Using this reporter, we observed induction of different [URE3] prion variants (“strains”) following overexpression of the N-terminal Ure2p prion domain (UPD) or full-length Ure2p. Full-length Ure2p induced two types of [URE3]: type A corresponds to conventional [URE3], whereas the novel type B variant is characterized by relatively high residual Ure2p activity and efficient curing by coexpression of low amounts of a UPD-green fluorescent protein fusion protein. 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 suggest that these strain variants result from different modes of aggregation of similar Ure2p monomers. We also demonstrate a procedure to counterselect against the [URE3] state.

ACKNOWLEDGMENTS

We are grateful to R. B. Wickner for generously providing strains and plasmids. We thank C. Cullin for providing strain YCC34 and J. H. Hegemann for providing the plasmids used for strain construction.

This work was supported by a research fellowship from the Deutsche Forschungsgemeinschaft to M.S. and grants from the National Institute of Aging and the National Institute of Neurological Diseases and Stroke of the National Institutes of Health (grant numbers AG02132, AG10770, and NS14069) and from the National Institute of General Medical Sciences (grant number GM59466 to I.H.).