Calcium-dependent antibiotics (CDA) are cyclic lipopeptides assembled by nonribosomal peptide synthetase (NRPS) enzymes. Active site modification of the 3-methyl glutamate activating adenylation (A) domain of the CDA NRPS enables the incorporation of synthetic 3-methyl glutamine into CDA. This provides the first example of how A-domains can be engineered to introduce synthetic "non-natural" amino acids into nonribosomal peptides. CI - Copyright (c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.