Exploring the biocombinatorial potential of benzoxazoles: generation of novel caboxamycin derivatives.

Background: The biosynthesis pathway of benzoxazole compounds caboxamycin and nataxazole have been recently elucidated. Both compounds share one of their precursors, 3-hydroxyanthranilate (two units in the case of nataxazole). In addition, caboxamycin structure includes a salicylate moiety while 6-methylsalycilate is the third scaffold in nataxazole. Pathways cross-talk has been identified in caboxamycin producer Streptomyces sp. NTK937, between caboxamycin and enterobactin pathways, and nataxazole producer Streptomyces sp. Tü6176, between nataxazole and coelibactin pathways. These events represent a natural form of combinatorial biosynthesis.

Results: Eleven novel caboxamycin derivatives, and five putative novel derivatives, bearing distinct substitutions in the aryl ring have been generated. These compounds were produced by heterologous expression of several caboxamycin biosynthesis genes in Streptomyces albus J1074 (two compounds), by combinatorial biosynthesis in Streptomyces sp. NTK937 expressing nataxazole iterative polyketide synthase (two compounds) and by mutasynthesis using a nonproducing mutant of Streptomyces sp. NTK937 (12 compounds). Some of the compounds showed improved bioactive properties in comparison with caboxamycin.

Conclusions: In addition to the benzoxazoles naturally biosynthesized by the caboxamycin and nataxazole producers, a greater structural diversity can be generated by mutasynthesis and heterologous expression of benzoxazole biosynthesis genes, not only in the respective producer strains but also in non-benzoxazole producers such as S. albus strains. These results show that the production of a wide variety of benzoxazoles could be potentially achieved by the sole expression of cbxBCDE genes (or orthologs thereof), supplying an external source of salicylate-like compounds, or with the concomitant expression of other genes capable of synthesizing salicylates, such as cbxA or natPK.