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Specificity of the Tyrosine-Phenylalanine Transport System in Bacillus subtilis

Department of Chemistry, Texas A & M University, College Station, Texas 77843, and Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77025

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-[¹⁴C]tyrosine and L-[¹⁴C]phenylalanine competitively were determined throughout a wide range of substrate and inhibitor concentrations. In this manner, the contributions of the side chain, the -amino group and the carboxyl group to uptake specificity were established. It is concluded that the positively charged -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.