In vitro and in vivo characterization of the cyclomarin/cyclomarazine prenyltransferase CymD revealed its ability to prenylate tryptophan prior to incorporation into both cyclic peptides by the nonribosomal peptide synthetase CymA. This knowledge was utilized to bioengineer novel derivatives of these marine bacterial natural products by providing synthetic N-alkyl tryptophans to a mutant of Salinispora arenicola CNS-205.