The genetic properties of the [URE3] non-Mendelian element of Saccharomyces cerevisiae suggest that it is a prion (infectious protein) form of Ure2p, a regulator of nitrogen catabolism. In extracts from [URE3] strains, Ure2p was partially resistant to proteinase K compared with Ure2p from wild-type extracts. Overexpression of Ure2p in wild-type strains induced a 20- to 200-fold increase in the frequency with which [URE3] arose. Overexpression of just the amino-terminal 65 residues of Ure2p increased the frequency of [URE3] induction 6000-fold. Without this “prion-inducing domain” the carboxyl-terminal domain performed the nitrogen regulation function of Ure2p, but could not be changed to the [URE3] prion state. Thus, this domain induced the prion state in trans, whereas in cis it conferred susceptibility of the adjoining nitrogen regulatory domain to prion infections.