Many species of Trichoderma have attracted interest as agents for the biological control of soil borne fungal pathogens of a range of crop plants. Research on the biochemical mechanisms associated with this application has focused on the ability of these fungi to produce enzymes which lyse fungal cell walls, and antifungal antibiotics. An important group of the latter are the non-ribosomal peptides called peptaibols. In this study Trichoderma asperellum, a strain used in biological control in Malaysia, was found to produce the peptaibol, trichotoxin. This type of peptide molecule is synthesised by a peptide synthetase (PES) enzyme template encoded by a peptide synthetase (pes) gene. Using nucleotide sequences amplified from adenylation (A-) domains as probes, to hybridise against a lambda FIXII genomic library from T. asperellum, 25 clones were recovered. These were subsequently identified as representative of four groups based on their encoding properties for specific amino acid incorporation modules in a PES. This was based on analysis of their amino acid sequences which showed up to 86% identity to other PESs including TEX 1.
Trichoderma spp. are regularly found as a constituent of the mycoflora of many soils and are noted for their antagonistic activity against bacteria and other fungi. This latter property is the basis for the widespread interest in their use in the biological control of soil-borne fungal plant pathogens. This antagonism is partly based on their ability to produce an impressive inventory of secondary metabolites. An important group of bioactive metabolites produced by Trichoderma spp. are the non-ribosomal peptides (NRPs), especially the peptaibols. A virulent antagonistic strain, T. asperellum, which had been used in biological control strategies in Malaysia and previously examined for mycolytic enzyme production, has been studied for its potential for peptaibol production. The present research demonstrated the ability of T. asperellum to produce at least two metabolites which were identified as acid trichotoxin 1704E (Ac-Aib-Gly-Aib-Leu-Aib-Gln-Aib-Aib-Aib-Ala-Ala-Aib-Pro-Leu-Aib-Iva-Glu-Vol) and neutral trichotoxin 1717A (Ac-Aib-Gly-Aib-Leu-Aib-Gln-Aib-Aib-Aib-Ala-Aib-Aib-Pro-Leu-Aib-Iva-Gln-Vol). Addition of free Aib to the culture medium enhanced the production of trichotoxins. Biological activity of these substances was investigated against Bacillus stearothermophilus. The general characteristics of peptaibols, also found in the trichotoxins, include the presence of high proportions of the uncommon amino acid Aib, the protection of the N- and C-termini by an acetyl group and reduction of the C-terminus to 2-amino alcohols, respectively, amphipathy and microheterogeneity.