Relation between the Activated Methyl Cycle (AMC) and AI-2 production in bacteria. The AMC is responsible for the generation of the major methyl donor in the cell, S-adenosyl-L-methionine (SAM) and the recycling of methionine by detoxification of S-adenosyl-L-homocysteine (SAH). LuxS takes part in this cycle by salvaging the homocysteine moiety from the cycle intermediate S-ribosyl-homocysteine (SRH). As a by-product of this reaction the direct AI-2 precursor 4,5-dihydroxy-2,3-pentadione (DPD) is formed. DPD undergoes further reactions to form distinct biologically active signal molecules generically termed AI-2. (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate (S-THMF-borate), the AI-2 signal of Vibrionales, is produced without the help on any known enzyme in the presence of boric acid (lower pathway), while in other bacteria (e.g., S. typhimurium) DPD rearranges spontaneously to form (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF) as AI-2 signal (upper pathway). CH3-THPG: N5-methyltetrahydropteroryl glutamate, CH3-THF: N5-methyltetrahydrofolate.