[Effect of ammonium ions on the uptake of L-leucine in Saccharomyces cerevisiae. Repression and inhibition of transport systems].

L-leucine entrance into Saccharomyces cerevisiae is mediated by the general amino acid permease, GAP and two transport systems, S1 and S2, kinetically characterized. S1 is a high-affinity, low-velocity transport system, operating at lower L-leucine external concentration (0.05-0.1 mM), while S2 is a low-affinity, high-velocity transport system, operating at higher L-leucine external concentration (1.0 mM). In cells grown in minimal medium containing ammonium as sole nitrogen source the values of L-leucine entrance and uptake are smaller than those in cells grown in L-proline containing medium. When GAP is repressed by ammonium, L-leucine entrance is mediate by systems S1 and S2. Both systems are inhibited by ammonium. When GAP is derepressed, in cells grown in L-proline medium, L-leucine is transported by systems S1 and GAP (lower L-leucine external concentration), and mainly by S2 (higher L-leucine external concentration). GAP is the largest system inhibited by ammonium.