Studies on the Mechanism of Escherichia coli Resistance to Ethionine. ∗

Abstract

Summary

E. coli resistant to ethionine shows a 6 to 8 hour longer lag period than sensitive cells when grown on a glucose salts medium without ethionine. The lag period was reduced to that of sensitive cells when methionine, ethionine, cystathionine or homocysteine was added to the growth medium. When ethyl-1-C¹⁴-ethionine was added to resistant cells there was a rapid increase of the tracer into both the amino acid pool and protein followed by a rapid decrease of label in both fractions. Formate, acetate, and CO₂ isolated from culture filtrates of resistant cells metabolizing ethyl-1-C¹⁴-ethionine were labeled. No significant differences were noted between the incorporation of C¹⁴-methionine by ethionine-resistant and sensitive cells. On the basis of these observations, it is concluded that ethionine resistance in E. coli is the result of the synthesis of enzymes capable of converting ethionine to methionine. The data indicate that ethionine is de-ethylated to the level of homocysteine followed by transmethylation to methionine.