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 Salmonella typhimurium LT2 have been separated into two components by detailed kinetic analysis; the K_m values for L-isoleucine are 1μM and 12μM, for L-valine 1.2μM and 29μM, and for L-leucine 0.3μM and 11μM, respectively. Two mutant strains, KA261 (ilvC8, ilvT3, ilv-261) and KA265 (ilvC8, ilvT3, ilv-265), having multiple lesions in the transport systems have been 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-261) 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 third (low-affinity-(2)) system (K_m: 88 to 118μM) 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, whereas 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 strains, CE4, KA261 and KA265. Transport of L-methionine, L-proline and L-threonine remains normal in the KA261 and KA265 mutants.