A nonpyrrolysine member of the widely distributed trimethylamine methyltransferase family is a glycine betaine methyltransferase

Pyrrolysine, the 22nd amino acid, is found in few proteins. One, the trimethylamine methyltransferase MttB, forms a small portion of a large family of proteins. Most in this family lack pyrrolysine and have no known activity. We show that one such protein, MtgB, is a glycine betaine methyltransferase, providing functional context that may explain the relationship between family members with and without pyrrolysine. Close relatives of MtgB are encoded in many of the abundant bacteria in the oceans, as well as different microbes undertaking symbioses ranging from plants to humans. This finding implies that MtgB might partake in a widespread and underappreciated pathway of GB metabolism contributing significantly to global carbon and nitrogen cycling as well as human health.