The prion model for [URE3] of yeast: spontaneous generation and requirements for propagation

The genetic properties of the non-Mendelian element, URE3], suggest that it is a prion (infectious protein) form of Ure2p, a mediator of nitrogen regulation in Saccharomyces cerevisiae. Into a ure2Delta strain (necessarily lacking URE3]), we introduced a plasmid overproducing Ure2p. This induced the frequent "spontaneous generation" of URE3], with properties identical to the original URE3]. Altering the translational frame only in the prion-inducing domain of URE2 shows that it is Ure2 protein (and not URE2 RNA) that induces appearance of URE3]. The proteinase K-resistance of Ure2p is unique to URE3] strains and is not seen in nitrogen regulation of normal strains. The prion-inducing domain of Ure2p (residues 1-65) can propagate URE3] in the absence of the C-terminal part of the molecule. In contrast, the C-terminal part of Ure2p cannot be converted to the prion (inactive) form without the prion-inducing domain covalently attached. These experiments support the prion model for URE3] and extend our understanding of its propagation.