Thiosine-resistant mutants of Escherichia coli K-12 with growth-medium-dependent lysyl-tRNA synthetase activity : I. Isolation and physiological characterization

Abstract

1. Thiosine-resistant mutants of Escherichia coli K-12 have been isolated which have decreased lysyl-tRNA synthetase (L-lysine: tRNA ligase (AMP), EC 6.1.1.6) activity. 2. These mutants have three unusual characteristics. 1. A. The activity of lysyl-tRNA synthetase is growth-medium-dependent, and in some strains is 15–20 times higher in an enriched medium than in minimal medium. 2. B. The activity of the lysyl-tRNA synthetase from the wild-type strain declines in stationary phase in AC broth, but the activity of the enzyme from mutant strains does not. 3. C. The lysyl-tRNA synthetase from the mutant strains is thermostable in comparison to the wild-type enzyme. 3. Alanine can stimulate the lysyl-tRNA synthetase activity in minimal medium. and thus can partially replace the requirement for an enriched growth medium.