Thioredoxins (Trx) 1 and 2, and three methionine sulfoxide reductases (Msr) whose activities are Trx-dependent, are expressed in Escherichia coli. A metB(1)trxA mutant was shown to be unable to grow on methionine sulfoxide (Met-O) suggesting that Trx2 is not essential in the Msr-recycling process. In the present study, we have determined the kinetic parameters of the recycling process of the three Msrs by Trx2 and the in vivo expression of Trx2 in a metB(1)trxA mutant. The data demonstrate that the lack of growth of the metB(1)trxA mutant on Met-O is due to low in vivo expression of Trx2 and not to the lower catalytic efficiency of Msrs for Trx2.