Properties of an Ethionine-Resistant (ER) Mutant of Ochromonas danica

SYNOPSIS. On prolonged incubation of ethionine-sensitive (ES) cells of Ochromonas danica in L-ethionine-containing media, growth was resumed by an ethionine-resistant (ER) mutant. Such mutants arise at random and are selected by the ethionine-containing medium. Ethionine resistance is not lost on repeated transfers thru ethionine-less media. ES cells incubated with ethionine form a large posterior vacuole before they disintegrate. Inhibition of reserve substance utilization is suggested to underlie growth inhibition of O. danica by ethionine. In ES cells incubated with ethionine, ¹⁴C uptake from labeled methionine, ethionine or serine is reduced by 65%. In ER cells the decrease in ¹⁴C uptake is 90%. This decrease in uptake of ethionine seems to be how ER O. danica evades growth inhibition by ethionine.