(A) Structures of FK506 and FK520. (B) Schematic representation of the FK506 biosynthetic cluster. The genes located on the left and right side from the FK506 core PKS region are presented in more detail. Putative regulatory gene homologues allN, fkbN and fkbR are represented by white arrows. Promoters used in the rppA reporter studies, deleted regions and RT-PCR amplified regions are marked.