Abstract
Three mutants, KA222, KA223 and KA224, derepressed in the transport of branched-chain amino acids were isolated as glycyl-L-leucine-resistant (Gler) strains from an isoleucine-valine-requiring mutant, KA931, of Salmonella typhimurium LT2. These Gler strains grow normally in minimal medium supplemented with L-isoleucine (10μg/ml), L-valine (20μg/ml) and large amounts of glycyl-L-leucine (1mM: 188μg/ml), where growth of the parent strain, KA931, is markedly inhibited. When cells were grown in the absence of glycyl-L-leucine, the Gler mutants incorporated wo- to three-fold higher amounts of L-isoleucine or L-leucine than did KA931. Although the transport activity of Gler strains was repressible by glycyl-L-leucine, the activity in the repressed state was equal to, or even higher than, the activity of KA931 in the unrepressed state. The increment of uptake in the Gler strains is mainly due to derepression of the low-affinity transport system. In the Gler strains, the transport of glycine, L-methionine, and L-proline was normal.
