The programming role of trans-acting enoyl reductases during the biosynthesis of highly reduced fungal polyketides

A novel polyketide synthase nonribosomal peptide synthetase (PKS-NRPS) gene cluster was isolated from Beauveria bassiania 992.05. The cluster encodes the enzymes responsible for the biosynthesis of the new 2-pyridone desmethylbassianin (DMB). DMB is structurally related to tenellin from B. bassiana 110.25 but it differs in chain length and degree of methylation. Despite these programming differences the 20 kb DMB biosynthetic gene cluster has 90% sequence identity to the tenellin gene cluster. Silencing of the PKS-NRPS gene, dmbS, resulted in total loss of DMB production. Co-expression of dmbS in Aspergillus oryzae with its cognate trans-acting enoyl reductase gene, dmbC, produced predesmethylbassianin A, the first isolable precursor in the biosynthetic pathway. Expression of dmbS with the tenellin trans-acting enoyl reductase gene, tenC, also resulted in the production of predesmethylbassianin A. Co-expression of tenS, the tenellin PKS-NRPS, with dmbC produced pretenellin A. These results show that the tenS and dmbS encoded PKS-NRPS contains the programme for polyketide biosynthesis, while the trans-acting ERs appear to control the fidelity of the programme. Expression of a hybrid synthetase in which the PKS of the tenellin synthetase was fused to the NRPS from DMBS produced prototenellins A to C, indicating that the NRPS does not act as a selecting gatekeeper to affect the PKS programme.