Competition and co-regulation of spirotoamide and tautomycetin biosynthesis in Streptomyces griseochromogenes, and isolation and structural elucidation of spirotoamide C and D

Spirotoamide A (1) and B (2) (Figure 1a), two spiroketal-containing polyketides, were recently isolated from a microbial metabolite fraction library of Streptomyces griseochromogenes JC82-1223 by searching for structurally unique compounds based on a spectral database. The carbon skeleton of spirotoamides was predicted to be assembled by a type I polyketide synthase (PKS) that utilizes acetyl-CoA as a starter unit and catalyzes decarboxylative condensation with four malonyl-CoAs, four methylmalonyl-CoAs and one ethylmalonyl-CoA as extender units. The spiroketal moiety of spirotoamides was speculated to be formed non-enzymatically, as no homologue was detected in the genome of S. griseochromogenes