Parallel Post-Polyketide Synthase Modification Mechanism Involved in FD-891 Biosynthesis in Streptomyces graminofaciens A-8890.

To isolate a key polyketide biosynthetic intermediate for the 16-membered macrolide FD-891 (1), we inactivated two biosynthetic genes coding for post-polyketide synthase (PKS) modification enzymes: a methyltransferase (GfsG) and a cytochrome P450 (GfsF). Consequently, FD-892 (2), which lacks the epoxide moiety at C8-C9, the hydroxy group at C10, and the O-methyl group at O-25 of FD-891, was isolated from the gfsF/gfsG double-knockout mutant. In addition, 25-O-methyl-FD-892 (3) and 25-O-demethyl-FD-891 (4) were isolated from the gfsF and gfsG mutants, respectively. We also confirmed that GfsG efficiently catalyzes the methylation of 2 and 4 in vitro. Further, GfsF catalyzed the epoxidation of the double bond at C8-C9 of 2 and 3 and subsequent hydroxylation at C10, to afford 4 and 1, respectively. These results suggest that a parallel post-PKS modification mechanism is involved in FD-891 biosynthesis.

Cloning of the biosynthetic gene cluster for naphthoxanthene antibiotic FD-594 from Streptomyces sp. TA-0256.

FD-594 is an unique pyrano[4',3':6,7]naphtho[1,2-b]xanthene polyketide with a trisaccharide of 2,6-dideoxysugars. In this study, we cloned the FD-594 biosynthetic gene cluster from the producer strain Streptomyces sp. TA-0256 to investigate its biosynthesis. The identified pnx gene cluster was 38143 bp, consisting of 40 open reading frames, including a minimal PKS gene, TDP-olivose biosynthetic genes, two glycosyltransferase genes, two methyltransferase genes and many oxygenase/reductase genes. Most of these enzymes coded in the pnx cluster were reasonably assigned to a plausible biosynthetic pathway for FD-594, in which an unique ring opening process via Baeyer-Villiger-type oxidation catalyzed by a putative flavin adenine dinucleotide (FAD)-dependent monooxygenase, is speculated to lead to the unique xanthene structure. To clarify the involvement of pnx genes in the FD-594 biosynthesis, a glycosyltransferase, PnxGT2, and a methyltransferase, PnxMT2, were characterized enzymatically with the recombinant proteins expressed in Escherichia coli. As a result, PnxGT2 catalyzed the triple olivose transfers to the FD-594 aglycon with TDP-olivose as the glycosyl donor to afford triolivoside. Surprisingly, in the PnxGT2 enzymatic reaction, tetraolivoside and pentaolivoside were significantly detected along with the expected triolivoside. To our knowledge, PnxGT2 is the first contiguous oligosaccharide-forming glycosyltransferase in secondary metabolism. Furthermore, addition of PnxMT2 and S-adenosyl-L-methionine into the PnxGT2 reaction mixture afforded natural FD-594 to confirm that the PnxGT2 reaction product was the expected regiospecifically glycosylated compound. Consequently, the identified pnx gene cluster appears to be involved in FD-594 biosynthesis.