Proline is an amino acid that is not only required for protein synthesis but can also serve as a nitrogen source. Although proline is the least-preferred nitrogen source for many lab strains of S. cerevisiae, it is the most abundant source of nitrogen in grapes, the natural environment of wild yeast (2). When more optimal sources of nitrogen are unavailable, S. cerevisiae cells degrade proline into glutamate via the proline utilization pathway, shown here (1, 4). In the mitochondria, proline is first converted into delta-1-pyrroline-5-carboxylate (P5C) by the PUT1 gene product, proline oxidase (EC 1.5.99.8). Then, P5C is processed by the delta-1-pyrroline-5-carboxylate dehydrogenase (EC 1.5.1.12) Put2p into glutamate (1).

PUT1 and PUT2 are both nuclear genes that are positively regulated by the transcriptional activator Put3p (4). Although Put3p is bound constitutively to the promoters of PUT1 and PUT2, transcriptional upregulation only occurs in the presence of proline and the absence of a preferred nitrogen source (2, 5). PUT1 and PUT2 are also subject to regulation by an effect known as nitrogen catabolite repression (NCR) (6, 7). NCR prevents utilization of proline as a nitrogen source if better nitrogen compounds such as ammonia, asparagine, or glutamine are present. PUT1 downregulation by NCR is mediated by the transcription factors Ure2p and Dal80p (7, 8). In the presence of proline and the absence of a preferred nitrogen source, NCR is released by the GATA transcriptional activators Gln3p and Gat1p (9, 8). Unrelated to NCR, PUT1 expression is also induced by rapamycin (9) and repressed by Spt10p (10). Spt10p is a global transcription factor that negatively regulates PUT1 by binding to a TATA element in the PUT1 promoter (10). Additionally, Put1p activity requires the presence of oxygen and this form of regulation is abolished when cells are respiratory deficient (11).

PUT1 gene mutations result in the inability of yeast to grow when proline is the sole nitrogen source. The lack of functional Put1p causes an accumulation of proline that has been linked to improved cell viability after freezing and dessication stress (12). Mutations in PUT1 orthologs lead to impaired motor and phototactic behavior in flies (13), impaired sensorimotor behavior in mice (14), and are associated with human (OMIM) psychiatric and behavioral phenotypes such as the autosomal recessive disease Type 1 hyperprolinemia (OMIM, 15) and schizophrenia (OMIM, 16).

Last updated: 2005-09-06