Jacob C. Carlson

The novel dienoyl tetramic acids tirandamycin C (1) and tirandamycin D (2) with activity against vancomycin-resistant Enterococcus faecalis were isolated from the marine environmental isolate Streptomyces sp. 307-9, which also produces the previously identified compounds tirandamycins A (3) and B (4). Spectroscopic analysis of 1 and 2 indicated structural similarity to 3 and 4, with differences only in the pattern of pendant oxygenation on the bicyclic ketal system. The isolation of these putative biosynthetic intermediates was enabled by their sequestration on an adsorbent resin during early stationary-phase fermentation.

The structurally intriguing bicyclic ketal moiety of tirandamycin is common to several acyl-tetramic acid antibiotics, and is a key determinant of biological activity. We have identified the tirandamycin biosynthetic gene cluster from the environmental marine isolate Streptomyces sp. 307-9, thus providing the first genetic insight into the biosynthesis of this natural product scaffold. Sequence analysis revealed a hybrid polyketide synthase-nonribosomal peptide synthetase gene cluster with a colinear domain organization, which is entirely consistent with the core structure of the tirandamycins. We also identified genes within the cluster that encode candidate tailoring enzymes for elaboration and modification of the bicyclic ketal system. Disruption of tamI, which encodes a presumed cytochrome P450, led to a mutant strain deficient in production of late stage tirandamycins that instead accumulated tirandamycin C, an intermediate devoid of any post assembly-line oxidative modifications.