A family of S-methylmethionine-dependent thiol/selenol methyltransferases - Role in selenium tolerance and evolutionary relation

Several plant species can tolerate high concentrations of selenium in the e nvironment, and they accumulate organoselenium compounds. One of these comp ounds is Se-methylselenocysteine, synthesized by a number of species from t he genus Astragalus (Fabaceae), like A. bisulcatus, An enzyme has been prev iously isolated from this organism that catalyzes methyl transfer from S-ad enosylmethionine to selenocysteine. To elucidate the role of the enzyme in selenium tolerance, the cDNA coding for selenocysteine methyltransferase fr om 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 prot ein was overproduced and purified. A functional analysis showed that the Ya gD protein catalyzes methylation of homocysteine, selenohomocysteine, and s elenocysteine with S-adenosylmethionine and S-methylmethionine as methyl gr oup donors. S-Methylmethionine was now shown to be also the physiological m ethyl group donor for the A. bisulcatus selenocysteine methyltransferase. A model system was set up in E, coli which demonstrated that expression of t he plant and, although to a much lesser degree, of the bacterial methyltran sferase gene increases selenium tolerance and strongly reduces unspecific s elenium incorporation into proteins, provided that S-methylmethionine is pr esent in the medium, It is postulated that the selenocysteine methyltransfe rase under selective pressure developed from an S-methylmethionine-dependen t thiol/selenol methyltransferase.