The mycobacterial members of this family are expressed from part of the ergothioneine biosynthetic gene cluster. EGTD is the histidine methyltransferase that transfers three methyl groups to the alpha-amino moiety of histidine, in the first stage of the production of this histidine betaine derivative that carries a thiol group attached to the C2 atom of an imidazole ring [1].

1: In vitro reconstitution of Mycobacterial ergothioneine biosynthesis. Seebeck FP; J Am Chem Soc. 2010;132:6632-6633. PMID:20420449