Enriched Growth Medium Research Articles

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

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.

Synchronization of the cell cycle of Tetrahymena by stravation and refeeding.

SYNOPSIS The present report describes a simple and useful method for synchronizing mass cultures of the ciliate Tetrahymena pyriformis. The method employs a nutritional approach which involves starvation of the cells in a non‐nutrient phosphate buffer followed by refeeding with an enriched nutritional growth medium. It takes 240 minutes after refeeding before the first cells start to divide. Radioautographic and DNA determinations taken together show that starved cells are stalled in the GI nuclear DNA condition and that essentially all of the cells replicate their DNA prior to their first cell division.