The third general transport system for branched-chain amino acids in Salmonella typhimurium.

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Abstract
Some properties of the LIV-III transport system, a common factor in the entry of branched-chain amino acids in Salmonella typhimurium, have been studied using a double mutant, KA266 (livA brnQ), which is defective in transport via either the LIV-I or LIV-II system. The LIV-III system operates with a low affinity (Km: >15 .mu.M) for L-isoleucine, L-leucine, and L-valine. Isoleucine transport via the LIV-III system is inhibited competitively by leucine or valine, but non-competitively by L-cysteine. Uptake of branched-chain amino acids into cells is partially repressed when the bacteria are grown in the presence of 2 mM glycyl-L-leucine or more. In the wild type, transport activity of the LIV-III system for isoleucine and leucine is not detected. The LIV-II system appears to show a low affinity for valine similar to the LIV-III system, hence the combined activity of these systems is expressed as the activity of the LIV-III system. The defect in transport via the LIV-II system leads to a great reduction in the transport of valine in the LIV-III system.