Translating biosynthetic gene clusters into fungal armor and weaponry

Abstract

Filamentous fungi are renowned for the production of a diverse array of secondary metabolites (SMs) where the genetic material required for synthesis of a SM is typically arrayed in a biosynthetic gene cluster (BGC). These natural products are valued for their bioactive properties stemming from their functions in fungal biology, key among those protection from abiotic and biotic stress and establishment of a secure niche. The producing fungus must not only avoid self-harm from endogenous SMs but also deliver specific SMs at the right time to the right tissue requiring biochemical aid. This review highlights functions of BGCs beyond the enzymatic assembly of SMs, considering the timing and location of SM production and other proteins in the clusters that control SM activity. Specifically, self-protection is provided by both BGC-encoded mechanisms and non-BGC subcellular containment of toxic SM precursors; delivery and timing is orchestrated through cellular trafficking patterns and stress- and developmental-responsive transcriptional programs.

Figure 1. Fungal BGCs can contain genes encoding one or more self-protective devices

Duplication (1) of the target of the BGC product can occur as a resistant form of the target that sustains activity even when the target is inhibited or in a form that is still sensitive but increases the target pool, allowing for some escape and, hence, activity. Other devices include enzymes that chemically modify the BGC product to fully or partially detoxify it (2) and transporters that export the BGC product outside of the cell (3).

Figure 2. Subcellular trafficking models for biosynthesis of aflatoxin, penicillin and trichothecene

(a) Aflatoxin synthesis is proposed to originate in the peroxisome (1); synthesis then proceeds from this organelle through multiple fusion events incorporating materials from the ER and ribosomally derived vesicles (2) to yield endosomes that fuse, eventually leading to large bodies containing substantial concentrations of aflatoxin (3). Contents of the aflatoxisomes are released to the environment in a yet unidentified manner. (b) Penicillin synthesis originates at the interface of the vacuole and cytosol (1). The first intermediate is released in the cytosol and processed by a second cytosolic enzyme (2), and the product from the second enzyme is transported to the peroxisome, where synthesis is completed (3). Two transporters, one on the vacuole and one on the peroxisome, are required for synthesis. The final product is released to the environment in a yet unidentified manner. (c) Trichothecene synthesis probably originates in a vacuole (1), with multiple steps associated with vesicle fusion events (2), with sufficient fusions leading to a toxisome (3). An in-cluster transporter is proposed to release the end product from the toxisome and outside of the cell but may also act to transport materials into the cell and between vesicles^,.

Figure 3. A transcriptional conduit from LaeA to BrlA regulates production of spore secondary metabolites

BrlA induction results in morphological development of the asexual sporulation structure (the conidiophore) and expression of spore-specific BGCs, including the fmq BGC genes responsible for making fumiquinazoline C. fmq genes are expressed only when BrlA is active, with FmqA localizing to a toxisome-like structure, FmqB and FmqC in the cytosol, and FmqD, the terminal enzyme, localizing to the spore cell wall.