[PSI] and [URE3] as yeast prions

[URE3] is a non‐Mendelian genetic element that mimics recessive mutations in the chromosomal URE2 gene making cells derepressed for nitrogen catabolic enzymes. [PSI] is a non‐Mendelian enhancer of readthrough of translational termination similar in its effects to some mutations in the chromosomal SUP35 gene. Three lines of evidence led to the proposal75 that both [URE3] and [PSI] are prions, infectious proteins analogous to the scrapie agent mediating transmissible spongiform encephalopathies of mammals. (1) Both [PSI] and [URE3] are reversibly curable. (2) [PSI] propagation requires SUP35 and [URE3] propagation requires URE2 with recessive chromosomal mutants having the same phenotypes as the presence of the respective dominant non‐Mendelian element. (3) Overproduction of Sup35p and Ure2p increases the frequency of cells acquiring [PSI] or [URE3], respectively.