Specificity of the Tyrosine-Phenylalanine Transport System in Bacillus subtilis

Abstract

l -Tyrosine and l -phenylalanine enter cells of Bacillus subtilis via a system of active transport that exhibits complex kinetic behavior. The specificity of the transport system was characterized both at low concentrations of transport substrate (where affinity for l -tyrosine or l -phenylalanine is high but capacity is low) and at high concentrations (where affinity is low but capacity is high). Specificity was not found to differ significantly as a function of either l -tyrosine or l -phenylalanine concentration. Kinetic analysis showed that the relationship between the uptake of l -phenylalanine and l -tyrosine is strictly competitive. Neither l -tyrosine nor l -phenylalanine uptake was competitively inhibited by other naturally occurring l -amino acids, indicating the importance of the phenyl side chain to uptake specificity. Hence, it is concluded that l -tyrosine and l -phenylalanine are transported by a common system that is specific for these two amino acids. The abilities of analogue derivatives of l -tyrosine and l -phenylalanine to inhibit the uptake of l -[ 14 C]tyrosine and l -[ 14 C]phenylalanine competitively were determined throughout a wide range of substrate and inhibitor concentrations. In this manner, the contributions of the side chain, the α-amino group and the carboxyl group to uptake specificity were established. It is concluded that the positively charged α-amino group contributes more significantly to uptake specificity than does the negatively charged carboxyl group. The recognition of a phenyl ring is an essential feature of specificity; other amino acids with aromatic side chains, such as the indole and imidazole rings of l -tryptophan and l -histidine, do not compete with l -tyrosine and l -phenylalanine for uptake. The presence of the p -hydroxy substitutent in the side chain (as in l -tyrosine) enhances the uptake of the aryl amino acid analogues investigated.