Abstract
Several plant species can tolerate high concentrations of selenium in
the environment, and they accumulate organoselenium compounds. One of
these compounds is Se-methylselenocysteine, synthesized by a number of
species from the genus Astragalus (Fabaceae), like A. bisulcatus, An
enzyme has been previously isolated from this organism that catalyzes
methyl transfer from S-adenosylmethionine to selenocysteine. To
elucidate the role of the enzyme in selenium tolerance, the cDNA coding
for selenocysteine methyltransferase from A. bisulcatus was cloned and
sequenced. Data base searches revealed the existence of several apparent
homologs of hitherto unassigned function, The gene for one of them, yagD
from Escherichia coli, was cloned, and the protein was overproduced and
purified. A functional analysis showed that the YagD protein catalyzes
methylation of homocysteine, selenohomocysteine, and selenocysteine with
S-adenosylmethionine and S-methylmethionine as methyl group donors.
S-Methylmethionine was now shown to be also the physiological methyl
group donor for the A. bisulcatus selenocysteine methyltransferase. A
model system was set up in E, coli which demonstrated that expression of
the plant and, although to a much lesser degree, of the bacterial
methyltransferase gene increases selenium tolerance and strongly reduces
unspecific selenium incorporation into proteins, provided that
S-methylmethionine is present in the medium, It is postulated that the
selenocysteine methyltransferase under selective pressure developed from
an S-methylmethionine-dependent thiol/selenol methyltransferase.
