Regulation of biosynthesis of aminoacyl-transfer R N A synthetases and of transfer-RNA in Escherichia coli Regulation de la biosíntesis de aminoacil-tRNA sintetasas y de RNA de transferencia en Escherichia coli

W e have isolated temperature resistant revertants from temperature sensitive E. coli strains containing either a thermolabile g lutaminyltRNA synthetase or leucyl-tRNA synthetase. Among the revertants which still contained the thermolabile leucyl-tRNA shynthetase w e found two classes of regulatory mutants (leuX and leuY) which have elevated levels of this enzyme. T h e leuX mutation specifies an operator-promoter region adjacent to the structural gene (leuS) for the enzyme. T h e leuY gene maps away from the leuS gene and codes for a protein. Us ing these mutants we demonstrated that the levels of leucyl-tRNA are related to the derepression of the leucine and isoleucine-valine operons. Among the revertants which still' contained the thermolabile g lutaminyltRNA synthetase we characterized three classes of mutants, glnT, glnU, and glnR. T h e glnT and glnU mutants contain elevated levels of t R N A 0 ' " , while the glnR mutant possesses elevated levels of glutaminyl tRNA synthetase. T h e level of glutamine synthetase, the enzyme responsible for the formation of glutamine, is also derepressèd in the gin T and glnR mutants.