Competition of Ethionine and Methionine for Aminoacyl-tRNA Formation in an In Vitro System from Ochromonas danica*

Aminoacyl-tRNA synthetase and tRNA were isolated from the chrysomonad Ochromonas danica. The mutual effect of methionine and ethionine, and the effect of other amino acids on methionyl- and ethionyl-tRNA formation, were tested in an in vitro system. The tRNA^Met had a similar accepting capacity for either methionine or ethionine. Ethionine and methionine, but none of the other amino acids tested, competed for the same aminoacyl-tRNA synthetase. The K_m of methionine was 0.88 × 10^–5 M, and that of ethionine 5 × 10^–4 M. Ethionine inhibited methionine binding; K_i 3.4 × 10^–4 M. The respective values in a similar system isolated from E. coli were 2.2 × 10^–5, 1.95 × 10^–3, and 1.95 × 10^–3.