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 tRNAHis (refs 1 and 2). This mutation allows constitutive synthesis of the histidine biosynthetic enzymes1,3. Recently, hisT mutants were found to have changes in tRNALeu (refs 1, 4 and 5), in tRNAIle (ref. 6), and to be partially derepressed for the isoleucine-valine biosynthetic enzymes5,6. 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.
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