Researchers have discovered the enzyme needed for synthesis of tenuazonic acid (TeA), a well-known toxin that is produced by multiple types of fungus and affects fruits, vegetables, rice, and other crops.
In their study published in Nature Communications, the authors describe how they found the gene for this enzyme, and reveal that its structure is unique among known enzymes.
TeA is known to be produced by at least three different plant pathogenic fungi, and is associated with spoiling of fruits, vegetables, and food-crops, as well as post-harvest decay.
Researchers identify the TeA biosynthetic gene from Magnaporthe oryzae by finding two TeA-inducing conditions of a low-producing strain.
They demonstrate that TeA is synthesized from isoleucine and acetoacetyl-coenzyme A by TeA synthetase 1 (TAS1). TAS1 is a unique non-ribosomal peptide synthetase and polyketide synthase (NRPS–PKS) hybrid enzyme that begins with an NRPS module.
In contrast to other NRPS/PKS hybrid enzymes, the PKS portion of TAS1 has only a ketosynthase (KS) domain and this domain is indispensable for TAS1 activity.
Phylogenetic analysis classifies this KS domain as an independent clade close to type I PKS KS domain. They demonstrate that the TAS1 KS domain conducts the final cyclization step for TeA release.
These results indicate that TAS1 is a unique type of NRPS–PKS hybrid enzyme.