One pathway, many compounds: heterologous expression of a fungal biosynthetic pathway reveals its intrinsic potential for diversity

Heterologous expression of genes from the proposed aspyridone biosynthetic cluster from Aspergillus nidulans in the host Aspergillus oryzae led to the production of eight different compounds in addition to aspyridone A 1, one of the previously observed products. The pathway genes were incapable of producing aspyridone B 2, the previously accepted final product of the pathway. Expression of restricted sets of genes in addition to the core polyketide synthase – nonribosomal peptide synthetase (PKS-NRPS) genes apdA and apdC revealed: that apdE encodes a cytochrome P450 enzyme with ring-expanding and unprecedented dephenylation activity; that apdB encodes an N-hydroxylase, an activity not previously suspected; that the productivity of ApdA and ApdC proteins appears to be significantly enhanced in the presence of the downstream ApdE oxidase; and no obvious chemical roles for ApdD and ApdG. Furthermore, the ApdC enoyl reductase appears to operate with different stereoselectivity in different PKS cycles. All of these features illustrate the inherent diversity of compounds potentially produced by the apd pathway and the high utility of a whole pathway expression strategy for investigating and revealing new biosynthetic chemistry in fungi.