Mutant tRNA His ineffective in repression and lacking two pseudouridine modifications

Abstract

TRANSFER RNA has been implicated in the regulation of a number of amino-acid biosynthetic operons1–4. Histidyl-tRNAHis has been shown to be involved in regulation of the histidine operon by analysis of six genes (hisO, hisR, hisS, hisT, hisU, hisW), mutation of which causes derepression of the enzymes of the histidine biosynthetic pathway in Salmonella typhimurium5–7. A class of derepressed mutants (hisR) has only about 55% as much tRNAHis as the wild type4 and in the one example sequenced, contains tRNAHIS with a structure identical to that of the wild type8. Studies of mutants of the gene for histidyl-tRNA synthetase (hisS) indicated that the derepressed phenotype was associated with defects in the charging of tRNAHISin vitro2. The amounts of charged and uncharged tRNAHis present in vivo during physiological derepression of the wild type and in the six classes of regulatory mutants, have been determined9. This work has shown that repression of the histidine operon is correlated directly with the concentration of charged histidyl-tRNAHisin vivo and not with the ratio of charged to uncharged or the absolute amount of uncharged tRNAHis. The derepression observed in mutants, of hisS (the gene for histidyl-tRNA synthetase), hisR (the presumed structural gene for the single species of tRNAHis) and hisU and hisW (genes presumably involved in tRNA modification) may be explained by the lower cellular concentration of charged tRNAHis which these mutants contain.