Abstract
Known SABATH methyltransferases, all of which were identified from seed plants, catalyze methylation
of either the carboxyl group of a variety of low molecular weight metabolites or the nitrogen moiety of
precursors of caffeine. In this study, the SABATH family from the bryophyte Physcomitrella patens was
identified and characterized. Four SABATH-like sequences (PpSABATH1, PpSABATH2, PpSABATH3, and PpSABATH4)
were identified from the P. patens genome. Only PpSABATH1 and PpSABATH2 showed expression in
the leafy gametophyte of P. patens. Full-length cDNAs of PpSABATH1 and PpSABATH2 were cloned and
expressed in soluble form in Escherichia coli. Recombinant PpSABATH1 and PpSABATH2 were tested for
methyltransferase activity with a total of 75 compounds. While showing no activity with carboxylic acids
or nitrogen-containing compounds, PpSABATH1 displayed methyltransferase activity with a number of
thiols. PpSABATH2 did not show activity with any of the compounds tested. Among the thiols analyzed,
PpSABATH1 showed the highest level of activity with thiobenzoic acid with an apparent Km value of
95.5 lM, which is comparable to those of known SABATHs. Using thiobenzoic acid as substrate, GC–
MS analysis indicated that the methylation catalyzed by PpSABATH1 is on the sulfur atom. The mechanism
for S-methylation of thiols catalyzed by PpSABATH1 was partially revealed by homology-based
structural modeling. The expression of PpSABATH1 was induced by the treatment of thiobenzoic acid. Further
transgenic studies showed that tobacco plants overexpressing PpSABATH1 exhibited enhanced tolerance
to thiobenzoic acid, suggesting that PpSABATH1 have a role in the detoxification of xenobiotic thiols.