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The inactivity of Ure2p, caused by either a ure2 mutation or the presence of the [URE3] prion, increases DAL5 transcription and thus enables Saccharomyces cerevisiae to take up ureidosuccinate (USA+). Rtg2p regulates transcription of glutamate-repressible genes by facilitation of the nuclear entry of the Rtg1 and Rtg3 proteins. We find that rtg2[DELTA] cells take up USA even without the presence of [URE3]. Thus, the USA+ phenotype of rtg2[DELTA] strains is not the result generation of the [URE3] prion but is a regulatory effect. Because rtg1[DELTA] or rtg3[DELTA] mutations or the presence of glutamate do not produce the USA+ phenotype, this is a novel function of Rtg2p. The USA+ phenotype of rtg2[DELTA] strains depends on GLN3, is caused by overexpression of DAL5, and is blocked by mks1[DELTA], but not by overexpression of Ure2p. These characteristics suggest that Rtg2p acts in the upstream part of the nitrogen catabolism regulation pathway. retrograde signaling | DAL5 | [URE3] | prion

P rotein-based inheritance and prions are discussed relative to Saccharomyces and Podospora spp. On the basis of genetic properties two non-Mendelian genetic elements of S. cerevisiae, called (URE3) and (PSI), are prposed to be prions, or infectious proteins of Ure2p and Sup35p, respectively. Topics include genetic criteria for a prion, a prion form of Ure2p affecting nitrogen catabolism, a prion form of Sup35p, a translation release factor, (Het-s)Hets prion control of vegetative incompatibility in Podospora, comparison of prion systems, and structure-based inheritance: cortical inheritance on Paramecium and other ciliates.

The products of the Saccharomyces cerevisiae CIN1, CIN2 and CIN4 genes participate in a nonessential pathway required for normal microtubule function. In this article, we demonstrate that the product of PAC2 also functions in this pathway. PAC2 deletion mutants displayed phenotypes and genetic interactions similar to those caused by cin1[Delta], cin2[Delta] and cin4[Delta]. These include cold-sensitive microtubule structures and sensitivity to the microtubule depolymerizing agent benomyl. Involvement in a common functional pathway is indicated by the observation that all double mutant combinations are viable and no more affected than any single mutant. In addition, extra copies of CIN1 were found to suppress the benomyl sensitivity of pac2[Delta], cin2[Delta] and cin4[Delta], but not that caused by other mutations that affect microtubule function. Cin1p and Pac2p were found to be related in sequence to mammalian proteins that aid in the folding of [Beta]-tubulin into an assembly-competent state. Alleles of CIN1 were identified that could suppress the benomyl sensitivity of cin4-4 in a highly specific fashion. Our findings suggest that the guanine nucleotide-binding Cin4p interacts with Cin1p and regulates its tubulin folding activity.