Mutants of Salmonella typhimurium Defective in Transport of Branched-Chain Amino Acids

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Five mutants, CD15, CD31, CE4, CE5, and CE14, defective in the transport of branched-chain amino acids were isolated as glycyl-l-isoleucine and glycyl-l-valine requirers from an isoleucine-valine-requiring mutant, KA931, of Salmonella typhimurium LT2. Although these transport mutants do not grow in minimal medium supplemented with isoleucine (10 μg/ml) and valine (20 μg/ml), where the parent strain, KA931, grows normally, they do when the supplements are increased 10-fold in concentration, and the growth rate becomes comparable to that of the parent strain. When the apparent Km and Vmax for uptake of isoleucine, valine, and leucine in the transport mutants were compared to those of KA931, the Km was generally lower in the mutants, and the Vmax for isoleucine and leucine decreased to one-fourth to one-seventh and for valine one-eighth to one-fifteenth of that in the parent. In all mutants except CE5, the transport of methionine, arginine, threonine, and glycine was normal. The transport of threonine in CE5 appeared to be slightly impaired. In addition to the original mutation in the ilvC locus, the transport mutant has a mutation at the ilvT locus, which is closely linked to proC and may be located on the side of proC proximal to purE. About 26-fold difference in values of the co-transduction is noted in reciprocal transductions between KA502 and CD15 strains.

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