Effect of L-ethionine on the expression of the SOS system in Escherichia coli.

The effect of L-ethionine, the ethyl analog of the essential amino acid methionine, on the SOS system of Escherichia coli was studied. This compound does not induce either inhibition of cell division nor cessation of cell respiration in a RecA+ Met+ RelA+ strain, nor in RecA+ Met- RelA+ or RecA+ Met- RelA- mutants. Nevertheless, L-ethionine blocks the expression of both cited SOS functions in a recA441 mutant when it is growing at the restrictive temperature of 42 degrees C. Furthermore, the inhibitory effect of the L-ethionine on the induction of the SOS system in this mutant is increased when the cells are preincubated for several hours in the presence of the analog, before the temperature shift. Moreover, cultures of the recA441 mutant incubated at 42 degrees C in the presence of both L-ethionine and L-methionine present the same behaviour as the cultures of this mutant growing at the same temperature but without either amino acid. On the other hand, L-ethionine does not have any effect on the expression of the two mentioned SOS functions when these are induced by UV-irradiation in a RecA+ strain even if this compound is added to the cells several hours before irradiation.