Multiple transport systems for branched-chain amino acids as studied by mutants of Salmonella typhimurium.

Abstract

The transport systems for entry of branched-chain amino acids in the wild-type strain of S. typhimurium LT2 were separated into 2 components by detailed kinetic analysis; the Km values for L-isoleucine are 1 .mu.M and 12 .mu.M, for L-valine 1.2 .mu.M and 29 .mu.M, and for L-leucine 0.3 .mu.M and 11 .mu.M, respectively. Two mutant strains, KA261 (ilvC8, ilvT3, ilv-261) and KA265 (ilvC8, ilvT3, ilv-265), having multiple lesions in the transport systems were isolated from strain CE4 (ilvC8, ilvT3), defective in the low-affinity-(1) system. Kinetic analysis of the transport for L-isoleucine shows that KA266 (ilvT3, ilv-267) and KA267 (ilvT3, ilv-265), which are ilvC8+ transductants of KA261 and KA265, respectively, are defective in both the high-affinity and the low-affinity-(1) systems, but retain the 3rd (low-affinity-(2) system Km 88-118 .mu.M) which is not detected in the wild-type and KA203 (ilvT3) strains. Strains KA261 and KA265 require glycyl-L-isoleucine, glycyl-L-valine, glycyl-L-leucine and Ca-pantothenate for growth, but the parental strain CE4 requires L-isoleucine and glycyl-L-valine. Glycyl-L-isoleucine and glycyl-L-valine can be replaced by high concentration of L-isoleucine and L-valine in stains, CE4, KA261 and KA265. Transport of L-methionine, L-proline and L-threonine remains normal in the KA261 and KA265 mutants.