The modular organization of McyA, McyB and McyC proteins. These three proteins catalyze 6 rounds of elongation and the final cyclisation of the heptapeptidyl microcystin intermediate. The McyB1 and McyC adenylation domains are responsible for the recognition and activation of the amino acids found at the X and Z positions in the microcystin respectively . The McyB1 and McyC condensation domains are responsible for peptide bond formation between this activated amino acid and the growing peptide chain . Each circle represents a NRPS enzymatic domain: A, aminoacyl adenylation; M, N-methyltransferase; T, Thiolation domain, C, condensation; E, epimerization; Te, thioesterase.