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Food plants are important sources of iodine, an essential nutrient required for the synthesis of thyroid hormone in humans. Understanding iodine metabolism in plants is important for tackling iodine deficiency, a serious micronutrient deficiency in the world. Elucidation of iodine metabolism in plants is also important for understanding biogeochemical iodine cycling because plants absorb iodine from the soil and emit considerable amounts of gaseous iodine compounds into the atmosphere. HAMLESS TO OZONE LAYER (HOL) family proteins found in plants have been reported to synthesize methyl halides, including methyl iodide, from halide ions and S-adenosyl-L-methionine (SAM). Here, we report the kinetic analysis of two rice HOL proteins, named OsHOL1 and OsHOL2. Recombinant OsHOL1 and OsHOL2 proteins synthesized methyl iodide from an iodide ion and SAM. Kinetic analyses of the recombinant proteins showed that methyltransferase activities toward iodide ions were highest among the examined substrates—bromide, chloride, and thiocyanate ions. These results suggested that OsHOL1 and OsHOL2 are involved in iodine metabolism in rice and contribute to methyl iodide emissions from rice.
