polymerization into melanin-like pigments. Evidence
of this can often be seen where two colonies of wood-
decay fungi meet and produce heavily melanized zone
lines(actually continuous plates of melanized cells)
in the region of contact (Fig. 11.24). When fungi are
isolated from either side of a zone line they are found
to belong to different species or, more commonly,
different strains of a single species that are somatically
incompatible with one another. Zone-lined wood
(also termed “spalted” wood) has become fashionable
for ornaments (Fig. 11.24).
Biotechnology of wood-decay fungi
Lignocellulose is abundant as a byproduct of the
wood-processing industries and also in crop residues,
so there is the potential to use it as a cheap commer-
cial substrate. For example if the cellulose could be
degraded to sugars, these could be used to produce fuel
alcohol by microbial fermentation, as an alternative
to fossil fuels. This prospect has stimulated research
on delignification, especially by the white-rot fungus
Phanerochaete chrysosporium which grows rapidly
in submerged liquid culture and, unusually for
Basidiomycota, produces abundant conidia. In near-
optimum culture conditions it can degrade as much
as 200 mg lignin per gram of mycelial biomass per day.
However, it also degrades and utilizes the cellulose
component of lignocellulose, defeating the object of the
exercise. Genetic engineering offers a potential solution
to this, and was made possible by the discovery
that Phanerochaeteproduces lignin peroxidase in the
early stationary phase of batch culture (Chapter 4). The
enzyme production was strongly promoted by nitrogen
limitation and further stimulated by addition of lignin.
By comparing the mRNA produced in these conditions
and in corresponding noninducing conditions, the gene
for lignin peroxidase was identified, then sequenced
and cloned into E. coli. The enzyme produced by the
recombinant bacterium acts on a range of lignin
“model compounds” in vitroso there is a prospect of
using high-yielding recombinant microorganisms for
delignification processes.
White-rot fungi and their enzyme systems also have
potential for bioremediation of land contaminated by
aromatic pollutants, and many other processes, discussed
by Ralph & Catcheside (2002).
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