Discovery of the parnafungins, antifungal metabolites that inhibit mRNA polyadenylation, from the Fusarium larvarum complex and other Hypocrealean fungi

Abstract

Evaluation of fungal fermentation extracts with whole cell Candida albicans activity resulted in the identification of a novel class of isoxazolidinone-containing metabolites named parnafungins. Chemical-genetic profiling with the C. albicans fitness test identified the biochemical target as inhibition of poly-adenosine polymerase, a component of the mRNA cleavage and polyadenylation complex. Parnafungins were discovered from fermentation extracts of fungi resembling F. larvarum isolated from plants, plant litter and lichens. Furthermore authentic strains of F. larvarum var. larvarum and F. larvarum var. rubrum could be induced to produce parnafungins and their degradation products in low titers. Relationships among strains of the F. larvarum complex (FLC), including parnafungin-producing strains, were examined by cladistic analyses of rDNA, mitochondrial rDNA, and two protein-coding genes, comparisons of antifungal activity and antifungal metabolite profiles, and morphological phenotypes. Integrated analyses of these data led to the conclusion that the diversity within the FLC exceeded the one-to-one correspondence between F. larvarum and its teleomorph Cosmospora aurantiicola. Based on multiple gene sequence analyses, strains of the FLC formed a monophyletic clade inclusive of the parnafungin-producing strains. The FLC, including newly discovered parnafungin-producing strains, could be resolved into at least six different lineages, possibly representing cryptic species, of which one was not fully resolved from F. larvarum var. rubrum. Fusarium larvarum var. rubrum represents a species distinct from var. larvarum. Finally we report that two other species from the Hypocreales, Trichonectria rectipila and Cladobotryum pinarense, are able to produce parnafungins and their open-ring forms.

• antibiotics
• brefeldin A
• chemotaxonomy
• Cladobotryum pinarense
• Cosmospora
• entomopathogenic fungi
• helvolic acid
• lichenicolous fungi
• secondary metabolites
• Trichonectria rectipila

We are grateful for the help of Kerry O’Donnell who offered advice, access to strains at the NRRL and suggested sets of primer sequences. Plant materials for isolation of fungi from the Republic of Georgia were collected in collaboration with Manana Khutsishvili, The Institute of Botany of the Georgian Academy of Science, Tbilisi, Georgia.