Nutrient regulation of oligopeptide transport in Saccharomyces cerevisiae

Small peptides (2-5 amino acid residues) are transported into Saccharomyces cerevisiae via two transport systems: PTR (Peptide TRansport) for di-/tripeptides and OPT (OligoPeptide Transport) for oligopeptides of 4-5 amino acids in length. Although regulation of the PTR system has been studied in some detail, neither the regulation of the OPT family nor the environmental conditions under which family members are normally expressed have been well studied in S. cerevisiae. Using a lacZ reporter gene construct fused to 1 kb DNA from upstream of the genes OPT1 and OPT2, which encode the two S. cerevisiae oligopeptide transporters, the relative expression levels of these genes were measured in a variety of environmental conditions. Uptake assays were also conducted to measure functional protein levels at the plasma membrane. It was found that OPT1 was up-regulated in sulfur-free medium, and that Ptr3p and Ssy1p, proteins involved in regulating the di-/tripeptide transporter encoding gene PTR2 via amino acid sensing, were required for OPT1 expression in a sulfur-free environment. In contrast, as measured by response to toxic tetrapeptide and by real-time PCR, OPT1 was not regulated through Cup9p, which is a repressor for PTR2 expression, although Cup9p did repress OPT2 expression. In addition, all of the 20 naturally occurring amino acids, except the sulfur-containing amino acids methionine and cysteine, up-regulated OPT1, with the greatest change in expression observed when cells were grown in sulfur-free medium. These data demonstrate that regulation of the OPT system has both similarities and differences to regulation of the PTR system, allowing the yeast cell to adapt its utilization of small peptides to various environmental conditions.