Two modes of metabolic regulation of lysyl-transfer ribonucleic acid synthetase in Escherichia coli K-12

Abstract

Lysyl-tRNA synthetase [EC 6.1.1.6] activity was compared in 3 independently isolated E. coli K-12 mutants of the enzyme S-adenosyl-L-methionine synthetase (metK mutants) and their isogenic parents. In all 3 cases, the activity of the lysyl-tRNA synthetase was elevated 2 to 4-fold in the mutant strains. Glycyl-L-leucine (3 mM) usually enhanced lysyl-tRNA synthetase activity 2-to 3-fold in wild-type cells but did not further stimulate the synthetase activity in metK mutants. By 2 other criteria, the lysyl-tRNA synthetase from wild-type cells grown with the peptide and from the metK mutant RG62, grown in minimal medium, were similar. These criteria are enhanced resistance to thermal inactivation and altered susceptibility to endogenous proteases when compared with the synthetase from wild-type cells grown in minimal medium. In a separate set of experiments, the activities of the lysyl-,arginyl-, seryl- and valyl-tRNA synthetases were measured in an isogenic pair of rel+ and rel strains of E. coli grown in a relatively poor growth medium (acetate) and in enriched medium. In the rel+ strain the level of all 4 synthetases was higher (2 to 4-fold) in the enriched medium as expected. In the rel strain the difference in the activities of the synthetases between the 2 media were diminished. In all 4 cases the activities of the synthetases were higher in acetate medium in the rel strain. Evidence is presented that these 2 modes of metabolic regulation act independently.