RESUMEN
Fungi have the potential to produce a large repertoire of bioactive molecules, many of which can affect the growth and development of plants. Genomic survey of sequenced biofertilizer fungi showed many secondary metabolite gene clusters are anchored by iterative polyketide synthases (IPKSs), which are multidomain enzymes noted for generating diverse small molecules. Focusing on the biofertilizer Trichoderma harzianum t-22, we identified and characterized a cryptic IPKS-containing cluster that synthesizes tricholignan A, a redox-active ortho-hydroquinone. Tricholignan A is shown to reduce Fe(III) and may play a role in promoting plant growth under iron-deficient conditions. The construction of tricholignan by a pair of collaborating IPKSs was investigated using heterologous reconstitution and biochemical studies. A regioselective methylation step is shown to be a key step in formation of the ortho-hydroquinone. The responsible methyltransferase (MT) is fused with an N-terminal pseudo-acyl carrier protein (ψACP), in which the apo state of the ACP is essential for methylation of the growing polyketide chain. The ψACP is proposed to bind to the IPKS and enable the trans MT to access the growing polyketide. Our studies show that a genome-driven approach to discovering bioactive natural products from biofertilizer fungi can lead to unique compounds and biosynthetic knowledge.