Characterization of the Biosynthetic Gene Cluster for the Ribosomally Synthesized Cyclic Peptide Epichloecyclins in Epichloe festucae

The various grass-induced epichloecyclins of the Epichloe spp. are ribosomally synthesized and post-translationally modified peptides (RiPPs), produced as small, secreted cyclopeptides from a single gene, gigA. Here, four clustered and coregulated genes (gigA, gigB, gigC, and kexB) with predicted roles in epichloecyclin production in Epichloe festucae were evaluated through gene disruption. Subsequent chemical analysis indicates that GigB is a DUF3328 domain-containing protein associated with cyclization of epichloecyclins; GigC is a methyltransferase enzyme responsible for N-methylation of desmethylepichloecyclins; and KexB is a subtilisin-like enzyme, partly responsible for the propeptide cleavage of epichloecyclin intermediates. Symbiotic effects on the host phenotype were not observed for gigA, gigC, or kexB mutants, although Delta gigB infection correlated with increased host tiller height and biomass, while only Delta kexB exhibited an effect on endophyte morphology. Disrupting epichloecyclin biosynthesis showed negligible influence on the biosynthesis of E. festucae-associated alkaloids. Epichloecyclins may perform other secondary metabolism functions in Epichloe and other fungi.

Automated summary

The study reveals that the grass-induced Epichloecyclins in Epichloe spp. are ribosomally synthesized and post-translationally modified peptides. Four clustered and coregulated genes, gigA, gigB, gigC, and kexB, are involved in epichloecyclin production. GigB is associated with cyclization, GigC is responsible for N-methylation, and KexB is involved in propeptide cleavage. Disrupting epichloecyclin biosynthesis has little influence on the biosynthesis of E. festucae-associated alkaloids.