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FUNGAL NUTRITION 117

can bind to the active site of cellobiohydrolase, and

competitively inhibitthe enzyme action. Thus, if

cellobiose accumulates then the rate of cellulose

breakdown will automatically be slowed, and this

regulation links the rate of breakdown to the rate

at which the fungus requires glucose for growth and

metabolism. In addition to this, the regulation of cel-

lulose breakdown is achieved by the common feedback

system termed catabolite repression, whereby genes

encoding the enzymes are repressed when more readily

utilizable substrates like glucose are available.

The presence of cellulose is known to induce the

synthesis of cellulase enzymes. It is difficult to imag-

ine that cellulose itself could be the inducer because

it is insoluble. Instead, cellulose-degrading fungi are

thought to produce very low (constitutive) levels of

cellulase enzymes so that the breakdown products

such as cellobiose can act as signals for enzyme

induction when cellulose is present in the environ-

ment. Cellobiose has been found to act as a weak

inducer of cellulases in some fungi, and a derivative

of cellobiose, termed sophorose, is a much stronger

inducer for Trichoderma reesei, the fungus used com-

monly as a model organism for studies of cellulase

action.

In summary, by a combination of gene repression

(by high levels of glucose or cellobiose), competitive

inhibitionof enzyme action (by the partial break-

down products of cellulose, such as cellobiose), and gene

induction(in the absence of glucose but presence

of low levels of cellobiose or its derivatives), the rate

of cellulose breakdown is matched closely to the rate

at which a fungus can use the sugars released from the

substrate.

Commercial and ecological aspects of

cellulases

The ability to degrade “crystalline” cellulose (cotton fab-

rics, etc.) is found in relatively few fungi, but several

fungi can degrade the “soluble substituted celluloses”

such as carboxymethyl cellulosein which about

30% of the glucose residues bear a substituent methyl

group. These soluble celluloses form gels in water and

are produced commercially for many purposes, such as

wallpaper pastes and paint thickeners. The fungi that

degrade these products secrete an endoglucanase but

not cellobiohydrolase, and they include several com-

mon leaf-surface saprotrophs such as Cladosporiumspp.

and Sydowia polyspora(Fig. 6.7).

This raises the question of why some fungi produce

only part of the cellulase enzyme complex if they

cannot degrade natural forms of cellulose. A likely

explanation was discovered more than 40 years ago by

Taylor & Marsh (1963) but has been largely ignored.

These workers found that several Pythiumspp. (which

are not generally considered to be cellulolytic fungi)

could degrade cotton fibres taken from unopenedcot-

ton bolls on cotton plants, but could not degrade the

cotton fibres once the cotton bolls had opened and the

fibres had air-dried. Simple rewetting of the dried

fibres did not render them degradable. But they could

be degraded if they were “swollen” by treatment with

KOH. The implication is that the cellulose in natural,

moist plant cell walls may be susceptible to attack, pro-

viding a substrate that several fungi can use. But the

cellulose chains bond together tightly when the tissues

have dried, and then only a restricted group of fungi

can degrade them.

Among the typical cellulose-degraders is Trichoderma

reesei, originally isolated from rotting cotton fabric. By

routine, repeated selection in laboratory conditions,

some strains of this fungus have been obtained that

can release as much as 30 g dry weight of cellulase

enzymes per liter of culture broth (Penttila et al.

1991). The gene for a cellobiohydrolase of this fungus

has been cloned into Saccharomyces cerevisiaeunder

the control of a constitutive promoter. The yeast

then secretes large amounts of a functional cellulase,

even in the absence of the substrate. Such enzymes,

whether produced by conventional or genetically

engineered strains, have significant commercial roles,

for example in abrading denim products.

Fig. 6.7Sydowia polyspora (Aureo-
basidium pullulans), one of the “sooty
moulds” of leaf surfaces. It has short
hyphal compartments and forms bud-
like conidia at the septa.