Abstract
Selenium-accumulating Astragalus spp. contain an enzyme which
specifically transfers a methyl group from S-methylmethionine to the
selenol of selenocysteine, thus converting it to a nontoxic, since
nonproteinogenic, amino acid. Analysis of the amino acid sequence of
this enzyme revealed that Escherichia coli possesses a protein (YagD)
which shares high sequence similarity with the enzyme. The properties
and physiological role of YagD were investigated. 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 for
growth, whereas a metE metH double mutant still grows on
S-methylmethionine. Upstream of yagD and overlapping with its reading
frame is a gene (ykfD) which, when inactivated, also blocks growth on
methylmethionine in a metE metH genetic background. Since it displays
sequence similarities with amino acid permeases it appears to be the
transporter for S-methylmethionine. Methionine but not
S-methylmethionine in the medium reduces the amount of yagD 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
physiological roles of the ykfD and yagD products appear to reside in
the acquisition of S-methylmethionine, which is an abundant plant
product, and its utilization for methionine biosynthesis.
