STRUCTURAL ORGANIZATION OF A MULTIFUNCTIONAL POLYKETIDE SYNTHASE INVOLVED IN THE BIOSYNTHESIS OF THE MACROLIDE IMMUNOSUPPRESSANT FK506

The immunosuppressant FK506 is a 23-membered macrocyclic polyketide pr oduced by several Streptomyces species. Sequencing of a 19.5-kb contig uous segment of DNA from the FK506 gene cluster of Streptomyces sp. MA 6548 revealed the presence of a single 19.3-kb open reading frame desi gnated fkbA. fkbA encodes a component of the FK506 polyketide synthase , a complex enzyme system which catalyzes synthesis of the polyketide portion of FK506. The predicted product of gene fkbA is a 630 660-Da p rotein (6420 amino acids) that contains 19 independent domains with a high degree of amino acid sequence similarity to the catalytic activit ies of known fatty acid synthases. The identified domains are arranged into four repeated modules with a linear organization precisely as th at of animal fatty acid synthase and type I polyketide synthase. Each module participates in one round of chain extension and subsequent pro cessing and thus FkbA polypeptide catalyzes four of the ten condensati on steps required for synthesis of the FK506 macrolactone ring. Disrup tion of fkbA results in the generation of an FK506 non-producing mutan t demonstrating direct involvement of fkbA in the biosynthesis of FK50 6.