Characterization of Natural Products from Actinobacteria of the Tübingen Strain Collection – Screening, Isolation & Structure Elucidation

Abstract

The phylum of Actinobacteria is well known for its ability to produce a variety of bioactive compounds with various areas of application. Thus, the Tübingen strain collection, which currently comprises more than 1000 selected Actinobacteria strains, has been chosen as a biological source for the discovery of new natural products. A unique tool was developed, allowing for the contemporaneous cultivation of 96 strains on agar in microtiter plates and subsequent execution of agar diffusion assays comprising different target organisms and various bioactivity readouts. As a proof of concept, the Tübingen strain collection was screened against the monitor strain Staphylococcus aureus PC322 which has been developed to identify compounds that interfere with agr, a key player of quorum sensing (QS) signal transduction. Within the screening, 82 strains showed QS inhibition activity, while 284 strains exhibited antibacterial activity. Two of the most promising strains from the screening have been analysed in regard to their bioactive compounds with Streptomyces sp. Tü2700 exhibiting QS inhibition activity, and Streptomyces sp. Tü2401 showing antibacterial activity. The oxazole polyene γ-lactam/β-lactone antibiotic oxazolomycin A, the highly antibacterial and cytotoxic enediyne compound C-1027 and the hydrophilic sideromycins albomycin δ1 and δ2 could be isolated. Moreover, xanthocidin and six new derivatives were identified from the endophytic Streptomyces sp. AcE210. The structural resemblance of the xanthocidins to methylenomycin A, B, and C, led to a hypothesis for the xanthocidin biosynthesis, which is similar to the biosynthesis of methylenomycin. The decisive difference is the substrate specificity for branched chain starter units such as isobu-tyryl-CoA or 2-methylbutyryl-CoA of the MmyC-corresponding KASIII in Streptomyces sp. AcE210. Isobutyryl-CoA is a degradation product of L-valine. Feeding studies with isotopically labelled [13C5]-L-valine confirmed that Streptomyces sp. AcE210 incorporates an isobutyryl-CoA starter unit, resulting in a branched side chain in xanthocidin. Moreover, analysis of the genome sequence of Streptomyces sp. AcE210 revealed a cluster of homologues to the mmy genes involved in methylenomycin biosynthesis.