Draft genome sequence of a sponge-derived Brevibacillus sp. TP-B0800, a producer of ulbactins with tumor cell migration inhibitory activity

Biosynthetic Investigations of Ulbactins Unveil Two Novel Thiazolinyl Imine Reductases Crucial for the Generation of Siderophore Activity

Siderophores are ferric ion chelating natural products secreted by microorganisms to survive in an iron-deficient environment. During screening for siderophores from marine bacteria, four interconvertible ulbactins C1-C4 and oxidized derivatives were identified in the bacterium Pseudoalteromonas flavipulchra S16. Iron-ion chelating activity assays revealed that the reduction of thiazoline rings is critical for the activity. The ulbactins biosynthetic gene cluster (ubt) was identified in the genome of P. flavipulchra S16 and validated via gene knockout and heterologous expression. A unique feature of ulbactin biosynthetic machinery involves UbtA, an atypical didomain enzyme (ASal-SalS) converting chorismate to salicyl-AMP. Two novel thiazolinyl imine reductases, UbtL and UbtM, were characterized as being essential for thiazolidine formation. Substitution of the native promoter of the ubt gene cluster with a constitutive promoter yielded new ulbactin variants and intermediate compounds, including unprecedented hydroxylated and dehydroalanine-containing ulbactins. In vitro reconstitution of ulbactins uncovered a distinctive synergistic catalytic mechanism between UbtL and UbtM for heterocycle reduction, which drives structural diversification during ulbactin biosynthesis.

Biologically Inspired Total Synthesis of Ulbactin F, an Iron-Binding Natural Product

Natural products from environmental microbiomes provide exquisite templates for elucidating biological activity in the search for new drugs. A recently discovered marine Brevibacillus sp. metabolite, ulbactin F, was found to inhibit tumor cell migration and invasion at IC50 < 3 μM. Herein, we disclose the first total synthesis of ulbactin F and epi-ulbactin F, which was modeled after the biosynthetic pathway. The scaffold bears structural similarity to siderophores of human pathogens but contains a novel tricyclic ring system derived from cysteine. We have found that ulbactin F forms low-affinity metal complexes, with a preference for Fe3+ and Cu2+, which may hint both at its environmental role and its antimetastatic mechanism of action.