Two prion-inducing regions of Ure2p are nonoverlapping

Ure2p of Saccharomyces cerevisiae normally functions in blocking utilizatio n of a poor nitrogen source when a good nitrogen source is available. The n on-Mendelian genetic element [URE3] is a prion (infectious protein) form of Ure2p, so that overexpression of Ure2p induces the de novo appearance of i nfectious [URE3]. Earlier studies defined a prion domain comprising Ure2p r esidues 1 to 64 and a nitrogen regulation domain included in residues 66 to 354. We find that deletion of individual runs of asparagine within the pri on domain reduce prion-inducing activity. Although residues 1 to 64 are suf ficient for prion induction, the fragment from residues 1 to 80 is a more e fficient inducer of [URE3]. In frame deletion of a region around residue 22 4 does not affect nitrogen regulation but does eliminate prion induction by the remainder of Ure2p. Larger deletions removing the region around residu e 224 and more of the C-terminal part of Ure2p restore prion-inducing abili ty. A fragment of Ure2p lacking the original prion domain does not induce [ URE3], but surprisingly, further deletion of residues 151 to 157 and 348 to 354 leaves a fragment that can do so. The region from 66 to 80 and the reg ion around residue 224 are both necessary for this second prion-inducing ac tivity. Thus, each of two nonoverlapping parts of Ure2p is sufficient to in duce the appearance of the [URE3] prion.