Isoleucine and valine metabolism in Escherichia coli. V. Antagonism between isoleucine and valine.

The influence of the relative concentrations of amino acids in culture media on the growth of microorganisms was first brought to the attention of microbiologists by Gladstone (1939). He observed that isoleucine, valine, or leucine added singly to a synthetic medium inhibited the growth of the anthrax bacillus whereas a mixture of the three stimulated growth. Although Gladstone was unable at that time to explain the nature of the imbalance, he did suggest two possibilities. The more obvious possibility was that because of their similarity in structure, an excess of one amino acid (for example, leucine) might block the incorporation of another (for example, valine) into bacterial protoplasm. The second possibility suggested was that the amino acid added in excess might prevent the synthesis of the second. To the knowledge of the authors, there have been no reports in the literature which serve as well documented examples of either of Gladstone's possibilities. The first possibility requires that the antagonism between the two amino acids be competitive. Unfortunately, most of the examples of competitive antagonism between pairs of structurally similar amino acids involve amino acids which are required for growth and, hence, must be supplied in the medium. In one such case, carefully analyzed by Prescott et al. (1953), it was concluded that the inhibition of L-serine utilization by L-alanine in the medium was occurring at the cell surface and not at some intracellular site where it is presumed the synthesis of protoplasm occurs. One might question, therefore, whether similar interpretations might eventually be applied to most other examples of antagonisms between amino acid pairs. On the other hand, if an inhibitory amino acid prevented the intracellular synthesis of a structurally similar amino acid (Gladstone's second 1 This work was supported by a grant from the United States Public Health Service (G-4015) and by funds from the Eugene Higgins Trust. possibility), one would expect the antagonism between the two amino acids to be noncompetitive. An ideal system for testing whether one or the other of Gladstone's interpretations is applicable would be the reversal by L-isoleucine of the inhibitory effect of L-valine on the growth of Escherichia coli strain K-12 (Bonner, 1946). Since the strain can synthesize both valine and isoleucine, the possibility that valine is acting at the cell surface (by preventing permeation of isoleucine) can be eliminated. In this paper, experiments are described which seem best interpreted by Gladstone's second possibility, namely, that valine inhibits the synthesis of isoleucine in E. coli strain K-12 rather than the utilization of isoleucine. These results are in contrast to those obtained using an isoleucine and valine requiring mutant of E. coli in which the antagonism seems to occur at the cell surface quite analogous to the L-serine-L-alanine antagonism described by Prescott et at. (1953).