S-methylmethionine metabolism in Escherichia coli

Selenium-accumulating Astragalus spp. contain an enzyme which specifically transfers a methyl group from S-methylmethionine to the selenol of selenocy steine, thus converting it to a nontoxic, since nonproteinogenic, amino aci d. Analysis of the amino acid sequence of this enzyme revealed that Escheri chia coli possesses a protein (YagD) which shares high sequence similarity with the enzyme. The properties and physiological role of YagD were investi gated. YagD is an S-methylmethionine: homocysteine methyltransferase which also accepts selenohomocysteine as a substrate. Mutants in yagD which also possess defects in metE and metH are unable to utilize S-methylmethionine f or growth, whereas a metE metH double mutant still grows on S-methylmethion ine. Upstream of yagD and overlapping with its reading frame is a gene (ykf D) which, when inactivated, also blocks growth on methylmethionine in a met E metH genetic background. Since it displays sequence similarities with ami no acid permeases it appears to be the transporter for S-methylmethionine. Methionine but not S-methylmethionine in the medium reduces the amount of y agD protein. This and the existence of four MET box motifs upstream of yfkD indicate that the two genes are members of the methionine regulon. The phy siological roles of the ykfD and yagD products appear to reside in the acqu isition of S-methylmethionine, which is an abundant plant product, and its utilization for methionine biosynthesis.