The hisT gene codes for an enzyme responsible for the conversion of uridine to pseudouridine (I) in the anticodon region of many tRNA species in Salmonella typhimurium. We have previously shown that a hisT mutant has tRNAHiU which lacks pseudouridine in this region and as a consequence has an altered chromatographic behavior. We show here a similar alteration in chromatographic behavior of all tRNALeu and one tRNAIte species from a hisT mutant. By contrast, tRNAVl, which contains no pseudouridine except for the one in the TICG sequence, is chromatographically unaltered in a hisT mutant.The absence of pseudouridine in the anticodon region of tRNA in hisT mutants has been previously shown to cause derepression of the histidine operon. We show here that in hisT mutants the regulation of the leucine and the isoleucine and valine operons is also affected: the enzymes of these operons are refractory to repression by the branched chain amino acids. Hoev-er, there is no difference 1 etween -hisT and wild type in the pattern of derepression caused by isoleucine or valine limitation and only a slight difference in the enzyme levels in cells grown on minimal medium.The alteration in the regulation of branched chain amino acid operons may also explain why hisT mutants are resistant to inhibition of growth by the amino acid analogues 5,5,5-trifluoroleucine, 6i-hydroxyleucine, and norleucine and by the oligopeptides glycylglycylnorleucine and norleucylnorleucine.hisT mutants, originally isolated as being derepressed for the histidine operon in Salmonella typhimurium (1, 2), have an altered tRNAHiS which has been sequenced and which differs only in having two uridines in place of two pseudouridines (I) in the anticodon region (3). In contrast, the pseudouridine in the "TICPu" loop was present in both the wild-type and mutant tRNAHiS (3). Direct evidence has been obtained that the hisT gene product is a pseudouridylation enzyme. An enzyme that can modify specific uridine residues of bulk hisT tRNA (4) has been identified and purified using wild-type strains of S. typhimurium and Escherichia coli. This activity is not detectable in extracts derived from hi8T strains and is heat-labile in temperature-sensitive hisT strains (4).Other tRNAs which have I in the anticodon loop also appear to have W replaced by U in the hisT mutant: tRNATYr, tRNALeU, and tRNAHiB species from hisT are altered in chromatographic behavior (3). hisT mutants are also resistant to some amino-acid analogs of histidine, leucine, and tyrosine (3). As the growth rate of hisT amber mutants was almost normal (2, 5), we concluded (3) that these Is in tRNA (about half the known tRNA species) may have a function in regulation rather than in protein synthesis.Histidyl-tRNAH1s is necessary for the repression of the histidine operon in S. typhimurium (5). Studies on the regulation of the leucine, isoleucine, and valine operons have also implicated the amino acyl tRNAs in regulation, although this evidence, involving the respective tRNA synthetases (6-10), was o...

No abstract