Biosynthesis of the methylthio side chain of caldariellaquinone

Both the methyl and sulphur groups of the methylthio portion of Caldariellaquinone (CQ), a benzothiophenquinone-containing quinone present in Sulfolobus acidocaldarius, have been shown to arise from methionine. Mass spectral analysis of the CQ isolated from cells grown in the presence of [34S-methyl-2H3]-L-methionine clearly showed, however, that the methylthio group of CQ is not derived as an intact unit from the methylthio group of methionine.