Sequences of stilboflavin C: towards the peptaibiome of the filamentous fungus Stilbella (= Trichoderma) flavipes

Filamentous fungi of the genus Stilbella are recognized as an abundant source of naturally occurring a-aminoisobutyric acid-containing peptides. The culture broth of Stilbella (Trichoderma) flavipes CBS 146.81 yielded a mixture of peptides named stilboflavins (SF), and these were isolated and separated by preparative TLC into groups named SF-A, SF-B, and SF-C. Although all three of these groups resolved as single spots on thin-layer chromatograms, HPLC analysis revealed that each of the groups represents very microheterogeneous mixtures of closely related peptides. Here, we report on the sequence analysis of SF-C peptides, formerly isolated by preparative TLC. HPLC coupled to QqTOF-ESI-HRMS provided the sequences of 10 16-residue peptides and five 19-residue peptides, all of which were N-terminally acetylated. In contrast to the previously described SF-A and SF-B peptaibols, SF-C peptaibols contain Ser-Alaol or Ser-Leuol, which are rarely found as C-termini, and repetitive Leu-Aib-Gly sequences, which have not been detected in peptaibols before. Taking the previously determined sequences of SF-A and SF-B into account, the entirety of peptides produced by S. flavipes (the 'peptaibiome') approaches or exceeds 100 non-ribosomally biosynthesized peptaibiotics.