Reduced maximal levels of derepression of the isoleucine-valine and leucine enzymes in his T mutants of Salmonella typhimurium

STRAINS of Salmonella typhimurium with a mutation in the hisT locus, produce an enzyme that cannot convert two uridine residues to pseudouridine in the anti-codon region of tRNA^His (refs 1 and 2). This mutation allows constitutive synthesis of the histidine biosynthetic enzymes^1,3. Recently, hisT mutants were found to have changes in tRNA^Leu (refs 1, 4 and 5), in tRNA^Ile (ref. 6), and to be partially derepressed for the isoleucine-valine biosynthetic enzymes^5,6. This suggests strongly that tRNA is part of the repression machinery for these enzymes. We report here that hisT strains are greatly limited in their ability to derepress the isoleucine-valine and leucine enzymes during leucine starvation. Thus, the hisT mutation alters regulation of these enzymes in two ways; it seems to cause a marked reduction in their maximal level as well as leading to a partial loss of repressibility.