Properties of a Dicarboxylic Amino Acid Transport-deficient Mutant of Streptococcus faecalis

Abstract

A mutant of Streptococcus faecalis has been isolated which has a 20-fold higher growth requirement for l -glutamic acid than the parent strain. The ability to utilize l -glutamine for growth is not affected. The mutant also has a greater dependence than the parent strain on an external supply of vitamin B6. The rate of glutamic acid transport at low extracellular concentrations (0.05 m m ) was greatly reduced in the mutant but was only moderately affected at higher concentrations (5 m m ). In contrast to the parent, in which kinetic studies revealed the operation of a high affinity and a low affinity system for glutamic acid transport, the mutant possessed a single catalytic component with kinetic constants equal to those of the low affinity system. These findings and the greatly elevated growth requirement for glutamic acid indicate that the high affinity system is inactive in the mutant. The transport of aspartic acid and d -glutamic acid by the mutant were similarly adversely affected, whereas essentially identical kinetic constants for glutamine, α-aminoisobutyric acid, and cycloleucine transport were obtained for the mutant and parent strains. The absence of a functional high affinity dicarboxylic amino acid transport system in the mutant allowed a study of the specific properties of the low affinity system. In contrast to the parent strain, the initial rate of uptake of isotopically labeled glutamate in the mutant was not stimulated by glucose at any concentration. However, the accumulation of large amounts of this amino acid on extended incubation was dependent on an energy source. Glutamine competitively inhibited glutamic acid uptake in the mutant. There was no evidence that the mutant produced an inhibitor of the high affinity transport component.