(A) Ribosome catalyzed amide bond formation. (B) Amide bond formation catalyzed by the PEARL TglB. (C) LanB enzymes glutamylate Ser/Thr residues in their substrate peptides and subsequently eliminate the glutamate to form dehydroamino acids. PEARL proteins have homology with the glutamylation domain of LanB proteins but lack the glutamate elimination domain. Dha, dehydroalanine; Dhb, dehydrobutyrine. (D) Proposed biosynthetic pathway towards 3–thiaglutamate.1 SAM, S-adenosyl-L-methionine. CysRS, cysteine tRNA synthetase. SAH, S-adenosyl-L-homocysteine.