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FUNGAL METABOLISM AND FUNGAL PRODUCTS 133

is argued that these intermediates are shunted into sec-
ondary metabolic pathways, whose products are either
exported from the cell or accumulate as (predomin-
antly) inactive compounds. Indeed, at one stage sec-
ondary metabolism was termed “shunt metabolism.”
It could then be argued that the genes encoding the
secondary metabolic pathways are free to mutate (cer-
tainly more so than those encoding basic metabolism)
and selection pressure would favor mutations that
lead to products that benefit a fungus.
For example, antibiotics could be useful in defense
of territory, mycotoxins as animal antifeedants,
melanin for protection against UV damage, sex hor-
mones for attracting partners, and flavor or odor
components of toadstools for attracting insects for
spore dispersal. If this view of secondary metabolism
is correct, then it seems that many fungi have still to
find useful roles for their secondary metabolites, or we
have yet to find them!
In any case, secondary metabolism is under tight
regulatory control. As noted above, secondary meta-
bolites typically are produced towards the end of the
exponential growth phase in batch cultures, when

growth is limited by a critical shortage of a particular

nutrient but when other nutrients are still available.

In continuous-culture systems, secondary metabolites

can be produced throughout the exponential growth

phase. The critical factor is that the genes encoding

secondary metabolism are repressed by high levels of

particular nutrients, but in chemostats the culture

medium can be designed so that these repressor sub-

strates are the growth-limiting nutrients, always present

at low concentration because they are utilized as soon

as they enter the chemostat (Chapter 4).

The pathways and precursors of

secondary metabolism

If we return to Fig. 7.1 at the start of this chapter, we

see that a few key intermediates of the basic metabolic

pathways provide the starting points for the pathways

of secondary metabolism. Some of the more important

examples are shown in Table 7.1.

The single most important secondary metabolic

pathway is the polyketide pathway, which seems to

Table 7.1Some secondary metabolites derived from different pathways and precursors.

Precursor

Sugars

Aromatic amino acids

Aliphatic amino acids

Organic acids

Fatty acids

Acetyl-CoA

Acetyl Co-A

Metabolites; representative organisms

Few, e.g. muscarine (Amanita muscaria), kojic acid (Aspergillusspp.)

Some lichen acids

Penicillins (P. chrysogenum, P. notatum)

Fusaric acid (Fusariumspp.)

Ergot alkaloids (Claviceps, Neotyphodium)

Lysergic acid (Claviceps purpurea)

Sporidesmin (Pithomyces chartarum)

Beauvericin (Beauveria bassiana)

Destruxins (Metarhizium anisopliae)

Rubratoxin (Penicillium rubrum)

Itaconic acid (Aspergillusspp.)

Polyacetylenes (Basidiomycota fruitbodies and hyphae)

Patulin (Penicillium patulum)

Usnic acid (many lichens)

Ochratoxins (Aspergillus ochraceus)

Griseofulvin (Penicillium griseofulvum)

Aflatoxins (A. parasiticus, A. flavus)

Trichothecenes (Fusarium spp.)

Fusicoccin (Fusicoccum amygdali)

Several sex hormones: sirenin, trisporic acids, oogoniol, antheridiol

Cephalosporins (Cephalosporiumand related fungi)

Viridin (Trichoderma virens)

Pathway

–

Shikimic acid

Various, including

peptide synthesis

TCA cycle

Lipid metabolism

Polyketide

Isoprenoid