Fig. 1: Overall architecture of the dimeric CalA3 protein.

a Polyketide examples. Shared double bonds formed by DH ___domain dehydrations are highlighted with red color. b Domain organizations for KS-AT-DH-KR-ACP arranged PKSs. Five Rap modules, three Fkb modules, and eight Amph modules are involved in the biosynthesis of rapamycin (immunosuppressant), FK-506 (immunosuppressant), and amphotericin B (antibiotic), respectively. Domain arrangement-shared CalA3 is responsible for the biosynthesis of calcimycin. c Cryo-EM map of CalA3 at overall resolution of 3.38 Å. d Atomic model of CalA3 is shown in front, top, bottom and side views, and the dimensions are indicated. The pseudo-twofold symmetry axis is indicated by a black line with an arrow. The two protomers are differentiated by the apostrophe symbol. e Schematic diagram illustrating the “∞” shape of the ___domain arrangement of CalA3. The linear ___domain organization is shown at an approximate sequence scale, and each ___domain is colored with a unique color. Abbreviations used for each ___domain: DD N-terminal docking ___domain, LD linker ___domain, KS β-ketoacyl synthase, AT acyltransferase, DH dehydratase, KR ketoreductase, ψKR ketoreductase that contains only half of the typical active KR structure, ACP acyl carrier protein.