Lunaemycins, new cyclic hexapeptide antibiotics from the cave moonmilk-dwellerStreptomyces lunaelactisMM109T

Abstract

AbstractStreptomyces lunaelactisstrains have been isolated from moonmilk deposits which are calcium carbonate speleothems used for centuries in traditional medicine for their antimicrobial properties. Genome mining revealed that these strains are a remarkable example of aStreptomycesspecies with huge heterogeneity regarding their content in biosynthetic gene clusters (BGCs) for specialized metabolite production. BGC 28a is one of the cryptic BGCs that is only carried by a subgroup ofS. lunaelactisstrains for whichin silicoanalysis predicted the production of nonribo-somal peptide antibiotics containing the non-proteogenic amino acid piperazic acid (Piz). Comparative metabolomics of culture extracts ofS. lunaelactisstrains either or not holding BGC 28a combined with MS/MS-guided peptidogenomics and1H/13C NMR allowed to identify the cyclic hexapeptide with the amino acid sequence (D-Phe)-(L-HO-Ile)-(D-Piz)-(L-Piz)-(D-Piz)-(L-Piz), called lunaemycin A, as the main compound synthesized by BGC 28a. Molecular networking further identified 18 additional lunaemycins, 14 of them having their structure elucidated by HRMS/MS. Antimicrobial assays demonstrated a huge bactericidal activity of lunaemycins against Gram-positive bacteria including multi-drug resistant clinical isolates. Our work demonstrates how accuratein silicoanalysis of a cryptic BGC can highly facilitate the identification, the structural elucidation, and the bioactivity of its associated specialized metabolites.