TRI17 gene encodes a PKS involved in the synthesis of the C-4 lateral side chain of fungal trichothecenes

Harzianum A and trichodermin are trichothecene compounds which have aroused great interest in last years because they are produced by several Trichoderma spp species. Many of these Trichoderma strains have the ability to act as important biocontrol agents against phytopathogenic fungi. In addition, some of them are biofertilizers, increasing plant growth and inducing plant defense response and even tolerance to abiotic stresses. However, trichothecenes have been studied in last decades due to their toxigenic properties against plants and all the animals analyzed, as well as for human beings. Additionally, many of the trichothecenes have antimicrobial activities. Most of the producer fungi are indeed important phytopathogens causing important losses in economically relevant crops. Thus, they are among the mycotoxins of greatest concern to food and feed safety.

Biosynthesis of Trichoderma trichothecenes has been almost totally characterized in recent years. Furthermore, in the present work a genome comparative strategy was used to identify the gene involved in the synthesis of the 2,4,6 octatriendioyl side chain of harzianum A. Thus, by comparison of Ta37 genome with genomes of other trichothecene producers, a PKS encoding gene (tentatively named TRI17), ortholog to PKS genes located in the TRI clusters of other fungi which produce trichothecenes with lateral side chains of polyketide nature (i.e. Stachybotrys, Myrothecium, Spicellum , Trichothecium), was selected as candidate to be involved in this step. A TRI17 knock-out (Dtri17) mutant was isolated which lacks production of HA, but with increased production of trichodermol, the intermediate where the 2,4,5 octatriendioyl side chain would be transferred, further supporting the hypothesis that Tri17 catalyzes its biosynthesis. In addition, Dtri17 mutation was also complemented with Myrothecium roridum TRI17, which indicates that synthesis of a part of the macrocyclic side chain moiety of the trichothecenes produced by this fungus is also catalyzed by this enzyme.