Since its first clinical appearance in 1989 [1] it has been well

established in medicine as an important immunosuppressant drug. The primary clinical utility of tacrolimus is prevention of graft rejection following organ and reconstructive tissue transplants and also treatment of skin diseases and eczema [2, 3]. In recent clinical studies FK506-derived compounds have also shown promise for treatment H 89 cell line of neurological disorders [4, 5]. A common feature of FK506 (Figure 1A), and its biogenetically and structurally related complex polyketides such as FK520 and rapamycin, is the involvement of large multifunctional polyketide synthase (PKS) / non-ribosomal peptide synthetase (NRPS) systems, comprising multi-fatty acid synthase-like domains arranged in sets of modules [6]. FK506 gene cluster from Streptomyces sp. MA6548 (ATCC53770) encoding the biosynthesis of this important learn more drug was partially sequenced by Merck Research Laboratories [7–10]. In recent years, two entire gene clusters from Streptomyces sp. KCTC 11604BP and Streptomyces kanamyceticus KCTC 9225 [11], and a partial sequence of the FK506 gene cluster from Streptomyces tsukubaensis NRRL 18488 [12] have been published, thus allowing for the first time a comparative analysis of gene clusters involved in the formation of FK506 by different Streptomyces strains. Figure 1 (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. Better understanding

of regulation of secondary metabolite biosynthesis could play a significant role in improvement of industrial strains, as has been exemplified in the past [13]. Regulation of secondary metabolism in actinomycetes is often diverse and complex and the production of medroxyprogesterone active natural products is linked to many environmental and physiological signals [14]. In addition to numerous pleiotropic regulatory genes present in genomes of secondary metabolite-producing actinomycete strains, most of gene clusters encoding secondary metabolite biosynthesis contain pathway-specific regulatory genes, such as the SARP (Streptomyces antibiotic regulatory protein) family regulators [15] or the LAL (large ATP-binding regulators of the LuxR family) family regulators [16, 17]. Like the SARP family, the LAL family gene-homologues with end-to-end similarity appear to be confined to the actinomycetes [18]. The production of many important polyketides or other secondary metabolites often remains relatively low and improving production titers of these low-yield compounds has been of great interest to the industry.