Genome-guided discovery of tropansamycins: antimicrobial pentaketide ansamycins

Although ansamycins are clinically important macrolactams, their pentaketide subset exhibits limited biological activities. Genome mining of rhizosphere-derived Streptomyces sp. LR53 uncovered a cryptic pentaketide ansamycin gene cluster (tpm). Activation via 3-amino-5-hydroxybenzoic acid (AHBA) feeding, promoter replacement, and deletion of the cytochrome P450 (CYP450) gene tpm16 enabled the discovery of seven novel metabolites, tropansamycins A-G (1-7), establishing the seventh distinct pentaketide ansamycin scaffold. Notably, congeners 5 and 6, predominantly accumulated in the Δtpm16 mutant, exhibited potent activity against gram-negative pathogenic bacterium Xanthomonas oryzae and gram-positive bacterium Staphylococcus aureus, with MICs ranging from 2 to 8 µg/mL and were toxic to the producer strain cultivated on LB agar medium. This reveals that the CYP450 enzyme Tpm16 functions as a modification enzyme modulating the bioactivity profile of pathway intermediates. These findings not only expand the structural diversity of ansamycins but also highlight the potential of these pathogen-active ansamycins as promising leads.