Identification of a Polyketide Synthase Required for Alternariol (AOH) and Alternariol-9-Methyl Ether (AME) Formation in Alternaria alternata

Abstract

Alternaria alternata produces more than 60 secondary metabolites, among which alternariol (AOH) and alternariol-9-methyl ether (AME) are important mycotoxins. Whereas the toxicology of these two polyketide-based compounds has been studied, nothing is known about the genetics of their biosynthesis. One of the postulated core enzymes in the biosynthesis of AOH and AME is polyketide synthase (PKS). In a draft genome sequence of A. alternata we identified 10 putative PKS-encoding genes. The timing of the expression of two PKS genes, pksJ and pksH , correlated with the production of AOH and AME. The PksJ and PksH proteins are predicted to be 2222 and 2821 amino acids in length, respectively. They are both iterative type I reducing polyketide synthases. PksJ harbors a peroxisomal targeting sequence at the C-terminus, suggesting that the biosynthesis occurs at least partly in these organelles. In the vicinity of pksJ we found a transcriptional regulator, altR , involved in pksJ induction and a putative methyl transferase, possibly responsible for AME formation. Downregulation of pksJ and altR caused a large decrease of alternariol formation, suggesting that PksJ is the polyketide synthase required for the postulated Claisen condensations during the biosynthesis. No other enzymes appeared to be required. PksH downregulation affected pksJ expression and thus caused an indirect effect on AOH production.

Role of the Alternaria alternata Blue-Light Receptor LreA (White-Collar 1) in Spore Formation and Secondary Metabolism

“…8). Among the 10 polyketide synthase-encoding genes, 2 were silent and only 8 were expressed (7). From those, we have chosen pksA, …”

“…because it is involved in melanin biosynthesis (29; R. Fetzner, M. Wenderoth, K. Seither, A. Herr, R. Fischer, submitted for publication), and pksJ, because it is involved in alternariol biosynthesis (7). In addition, two genes of unknown function, pksC and pksD, were chosen.…”

“…Alternariol biosynthesis requires the activity of a polyketide synthase, 10 of which were identified recently in the genome. One of them, PksJ, was identified to catalyze the initial steps of alternariol biosynthesis (7). It was already reported 30 years ago that blue light reduced alternariol production in A. alternata (8).…”

Natural Dibenzo-α-Pyrones and Their Bioactivities

“…We know very little about the biosynthesis of dibenzo-α-pyrones in living organisms, including their genetics, biochemistry and biosynthetic pathways [2,25,65,66]. In plants, gallic acid (54), which is biosynthesized via the shikimate pathway [67], was considered as the precursor of ellagitannins [68].…”

“…However, in microorganisms, dibenzo-α-pyrones are biosynthesized via the polyketide pathway [3,25]. Polyketide synthase (PKS) is one of the postulated core enzymes in the biosynthesis of 6H-dibenzo[b,d]pyran-6-ones (i.e., alternariol, AME) in Alternaria alternata [66]. In a draft genome sequence of A. alternata, 10 putative PKS-encoding genes were identified.…”