112 CHAPTER 6
of any sugar or sugar derivative depends on the pres-
ence of appropriate membrane transport proteins, and
these have different substrate-specificities. Fungi usu-
ally have a constitutive transport protein for glucose.
It transports glucose in preference to other sugars in a
mixture, but it has a relatively low binding specificity
and so, in the absence of glucose, it will transport some
other sugars. The uptake of these sugars leads to the
induced synthesis of their specific carrier proteins.
This can be demonstrated easily when Saccharomyces
cerevisiaeis grown in a shaken liquid culture medium
containing a disaccharide such as lactose as the sole
carbon source (Fig. 6.2). The fungus degrades lactose
to glucose and galactose by means of a wall-bound
enzyme, ββ-galactosidase, then the fungus grows
rapidly by taking up the glucose preferentially, while
galactose remains in the culture medium. When the
glucose is depleted the growth rate slows for about 30
minutes while a galactose carrier protein is synthesized,
and then the growth rate increases again, using galactose
as the substrate. This type of biphasic growth curve (Fig.
6.2) is termed a diauxic growth curve. Sucrose is used
in a similar way: it is cleaved to glucose and fructose
by the wall-bound enzyme invertase, then the glucose
is taken up before fructose. However, a few fungi can-
not use sucrose as a substrate (e.g. Rhizopus nigricans,
Zygomycota; Sordaria fimicola, Ascomycota) because
they do not produce invertase.
The utilization of polymers requires the release of
specific enzymes. For example, many fungi can utilizeFig. 6.1Some of the major carbon substrates of fungi.
Fig. 6.2A diauxic growth curve of a fungus grown
on the disaccharide lactose: a, initial phase of growth on
glucose; b, temporary slowing of growth while a galac-
tose transporter is inserted in the membrane; c, resumed
growth on galactose.