Biosynthesis of 6‐Hydroxymellein Requires a Collaborating Polyketide Synthase‐like Enzyme

Abstract

The polyketide synthase (PKS)-like protein TerB, consisting of inactive dehydratase, inactive C-methyltransferase, and functional ketoreductase domains collaborates with the iterative non reducing PKS TerA to produce 6-hydroxymellein, a key pathway intermediate during the biosynthesis of various fungal natural products. The catalytically inactive dehydratase domain of TerB appears to mediate productive interactions with TerA, demonstrating a new mode of transinteraction between iterative PKS components.

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products

“…Both 22 and 23 , but not 8 , are slowly converted into their C‐8 hydroxyl‐derivatives including 24 by endogenous enzymes in A. oryzae , supported by feeding studies (Figure S18, S81‐S85). 6‐HM 2 in turn is hydroxylated at C‐6 to give 25 upon prolonged incubation which is also consistent with previous observations [10,17] . Supplementation of the fermentation medium with sodium bromide (50 μg⋅mL −1 ) results in the successful formation of mono‐ and dibrominated derivatives of 2 as well as mixed chloro‐bromo species based on distinct isotope ratios in the respective mass spectra (Figure S19).…”

“…We therefore generated A. oryzae transformants co‐expressing either the truncated ( terC t ) or the extended sequence of terC with terAB following an established protocol [24] . TerA and TerB produce 6‐HM 2 [25] that is the putative substrate for TerC [3,10] . Expression of the truncated terC t with terAB led to no change over expression of terAB alone.…”

“…TerB is a tridomain protein with an N‐terminal catalytically inactive dehydratase (ψDH), a central catalytically inactive C ‐methyltransferase‐ (ψ C‐ MeT) and a functional C ‐terminal keto‐reductase (KR) [10] . Interestingly, RslB, that is ca.…”

“…300 amino acid residues shorter than TerB, does not contain the central ψ C‐ MeT domain (InterPro [32] analysis). Since TerA and TerB interact during the biosynthesis of 6‐HM 2 in vivo we were interested whether any of these enzymes could be exchanged with its homologous protein RslA or RslB, respectively [10] . We therefore generated two transformant strains of A. oryzae expressing either terA + rslB or rslA + terB .…”

“…6‐HM 2 is an intermediate during the biosynthesis of terrein 1 and related (poly‐chlorinated) natural products such as 5 [3,17] . In the terrein pathway lactone 2 is formed by collaboration between an nrPKS and a multidomain protein with catalytically active KR‐domain in vivo [10] . Bioinformatic analysis allowed the identification of a related BGC in the marine‐derived Roussoella sp.…”