The [URE3] prion is an aggregated form of Ure2p that can be cured by overexpression of Ure2p fragments

The [URE3] nonchromosomal genetic element is a prion of Ure2p, a regulator of nitrogen catabolism in Saccharomyces cerevisiae. Ure2p(1-65) is the prio n domain of Ure2p, sufficient to propagate [URE3] in vivo. We show that ful l length Ure2p-green fluorescent protein (GFP) or a Ure2p(1-65)-GFP fusion protein is aggregated in cells carrying [URE3] but is evenly distributed in cells lacking the [URE3] prion. This indicates that [URE3] involves a self -propagating aggregation of Ure2p. Overexpression of Ure2p(1-65) induces th e de novo appearance of [URE3] by 1,000-fold in a strain initially [ure-o], but cures [URE3] from a strain initially carrying the [URE3] prion. Overex pression of several other fragments of Ure2p or Ure2-GFP fusion proteins al so efficiently cures the prion. We suggest that incorporation of fragments or fusion proteins into a putative [URE3] "crystal" of Ure2p poisons its pr opagation.