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Salmonella typhimurium strain CV123 (ara-9 gal-205 flrB1 ), isolated as a mutant resistant to trifluoroleucine, has derepressed and constitutive levels of enzymes forming branched-chain amino acids. This strain grows more slowly than the parent at several temperatures, both in minimal medium and nutrient broth. It overproduces and excretes sizeable amounts of leucine, valine, and isoleucine in comparison with the parental strain. BothleuS (coding for leucyl-transfer ribonucleic acid [tRNA]synthetase) andflrB are linked tolip (min 20 to 25) by P1 transduction, whereas onlyleuS is linked tolip by P22 transduction. Strain CV123 containing an F′lip + episome fromEscherichia coli has repressed levels of leucine-forming enzymes, indicating thatflrB + is dominant toflrB . Leucyl-tRNA synthetase from strain CV123 appears to be identical to the leucyl-tRNA synthetase in the parent. No differences were detected between strain CV123 and the parent with respect to tRNA acceptor activity for a number of amino acids. Furthermore, there was no large difference between the two strains in the patterns of leucine tRNA isoaccepting species after fractionation on several different columns. Several otherflrB strains exhibited temperature-sensitive excretion of leucine, i.e., they excreted leucine at 37 C but not 25 C. In one such strain, excretion at 37 C was correlated with derepression of some enzymes specified byilv andleu . These latter results suggest thatflrB codes for a protein.

flrB, a Regulatory Locus Controlling Branched-Chain Amino Acid Biosynthesis in Salmonella typhimurium