Involvement of S-adenosylmethionine-dependent halide/thiol methyltransferase (HTMT) in methyl halide emissions from agricultural plants: isolation and characterization of an HTMT-coding gene from Raphanus sativus (daikon radish)

Background: Biogenic emissions of methyl halides (CH3Cl, CH3Br and CH3I) are the major source of these compounds in the atmosphere; however, there are few reports about the halide profiles and strengths of these emissions. Halide ion methyltransferase (HMT) and halide/thiol methyltransferase (HTMT) enzymes concerning these emissions have been purified and characterized from several organisms including marine algae, fungi, and higher plants; however, the correlation between emission profiles of methyl halides and the enzymatic properties of HMT/HTMT, and their role in vivo remains unclear. Results: Thirty-five higher plant species were screened, and high CH3I emissions and HMT/HTMT activities were found in higher plants belonging to the Poaceae family, including wheat (Triticum aestivum L.) and paddy rice (Oryza sativa L.), as well as the Brassicaceae family, including daikon radish (Raphanus sativus). The in vivo emission of CH3I clearly correlated with HMT/HTMT activity. The emission of CH3I from the sprouting leaves of R. sativus, T. aestivum and O. sativa grown hydroponically increased with increasing concentrations of supplied iodide. A gene encoding an S-adenosylmethionine halide/thiol methyltransferase (HTMT) was cloned from R. sativus and expressed in Escherichia coli as a soluble protein. The recombinant R. sativus HTMT (RsHTMT) was revealed to possess high specificity for iodide (I-), bisulfide ([SH](-)), and thiocyanate ([SCN](-)) ions. Conclusion: The present findings suggest that HMT/HTMT activity is present in several families of higher plants including Poaceae and Brassicaceae, and is involved in the formation of methyl halides. Moreover, it was found that the emission of methyl iodide from plants was affected by the iodide concentration in the cultures. The recombinant RsHTMT demonstrated enzymatic properties similar to those of Brassica oleracea HTMT, especially in terms of its high specificity for iodide, bisulfide, and thiocyanate ions. A survey of biogenic emissions of methyl halides strongly suggests that the HTM/HTMT reaction is the key to understanding the biogenesis of methyl halides and methylated sulfur compounds in nature.