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The nucleotide sequence of the leuD gene of Salmonella typhimurium and of the downstream flanking region are presented. S1 mapping experiments identified 3' endpoints of leu mRNA 140 and 285 nucleotides downstream of the UAA stop codon of leuD mRNA. Experiments employing pulse-labeled RNA suggest that these endpoints result from transcription termination rather than RNA processing. Our results indicate that the organization of the 3' non-translated region of the leu operon from S. typhimurium resembles that of the trp operon of Escherichia coli. Further, our results suggest that the leu operon of S. typhimurium does not contain structural genes other than those identified by genetic experiments, i.e. leu, A,B,C and D.

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Salmonella typhimurium strain CV123 (ara-9 gal-205 flrBl), 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. Both leuS (coding for leucyl-transfer ribonucleic acid [tRNA]synthetase) and flrB are linked to lip (min 20 to 25) by P1 transduction, whereas only leuS is linked to lip by P22 transduction. Strain CV123 containing an F' lip+ episome from Escherichia coli has repressed levels of leucine-forming enzymes, indicating that flrB+ is dominant to flrB. 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 other flrB 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 by ilv and leu. These latter results suggest that flrB codes for a protein. 942 on July 16, 2020 by guest