Biosynthetic Characterization of Bacillibactin in Thermophilic Bacillaceae and its Potential for <i>in Vitro</i> Mutational Synthesis

Bacillibactin (BB) is a microbial siderophore produced by Bacillus species. BB is biosynthesized from 2,3-dihydroxybenzoic acid (2,3-DHB), Gly, and L-Thr by nonribosomal peptide synthetase (NRPS) enzymes DhbE, DhbB, and DhbF. The biosynthetic gene cluster (dhb) is also conserved in some strains of thermophilic genera, Geobacillus, Anoxybacillus and Parageobacillus. However, the production of BB from these thermophilic bacteria has not been characterized. Here, we report in vivo and in vitro characterization of BB biosynthesis in Parageobacillus sp. KH3-4 which grows at 65 °C. We confirmed BB production in this thermophilic bacterium and the gene cluster active. In vitro enzymatic analysis revealed that 4'-phosphopantetheinyltransferase (PPTase) encoded in the same gene cluster is responsible for the post-translational maturation of carrier proteins. DhbE and DhbF showed substrate preference to 2,3-DHB and Gly and L-Thr, respectively, consistent with the chemical structure of BB. With the purified enzymes, we successfully reconstituted the NRPS assembly line in vitro. In addition, using chemically synthesized acyl-N-acetylcysteamine substrate analogues, BB analogues possessing methylbenzoyl groups instead of 2,3-DHB were detected. This study provides a new insight into secondary metabolism in thermophiles, and it expands the temperature limitation of NRPS enzymes.