Figure 1.

NCR. In the presence of a good nitrogen source, such as ammonia, the cytoplasmic protein Ure2p binds to Gln3p and prevents its entrance into the nucleus. Gln3p is a DNA-binding positive transcription regulator required for the expression of multiple genes (such as DAL5) involved in transport or assimilation of poor nitrogen sources (such as allantoate). Dal5p, the allantoate transporter, also takes up the chemically similar USA, an intermediate in the uracil biosynthetic pathway. Cells blocked in aspartate transcarbamylase and supplied with USA in place of uracil on ammonia-containing medium can grow only if the NCR pathway is inactive. A ure2 mutation or the presence of the [URE3] prion results in inactivity of Ure2p and constitutive activity of Gln3p and thus the USA+ phenotype. Mks1p acts as an inhibitor of Ure2p. The role tentatively assigned to Rtg2p in NCR is based on work presented in this study.