A regulatory livR mutation affecting high- and low-affinity transport systems for branched-chain amino acids in Salmonella typhimurium.

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A regulatory mutant, KA600 (ilvC8 livR3::Tn10), derepressed in the transport of branched-chain amino acids, was isolated from a strain KA931 (ilvC8) of Salmonella typhimurium LT2, after mutagenesis with tetracycline resistance transposon, Tn10. KA600 still carried P22-phage genome used as a vector in mutagenesis, and when grown on an agar medium, it segregated frequently phage-free clones possessing either a tetracycline-resistance (tetR) or a tetracycline-sensitive (tets) trait. Two of these segregant clones, KA601 (ilvC8 livR3::Tn10) and KA602 (ilvC8 livR3), were not different from KA600 in the mode of L-isoleucine transport. Levels of L-isoleucine and L-leucine uptake by KA610 (livR3::Tn1O), an Ilv+ transductant of KA601, were increased 1.4- to 2.0-fold over those of the wild-type. These uptake abilities were not repressed at all by 5mM glycyl-L-leucine. Activity of the branched- chain amino acid binding protein(s) of KA602 released by osmotic-shock was several times higher than that of KA931, although the activity of the former was repressible by glycyl-L-leucine to some extent. The livR locus was mapped in the region of 75 to 77 units on the genetic map of S. typhimurium. Nature of livR mutation distinct from a similar regulatory mutation, liv-231 is discussed.
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