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detoxify tomatine, and tomatinase (from S. lycopersici)

had negligible ability to detoxify avenacin. So it seems

that these pathogenic fungi have evolved saponin-

detoxifying enzymes with quite specific activity

against the saponins of their hosts.

Yeast killer systems

With this topic we return to the role of viral dsRNA.

Individual species of at least eight genera of yeasts

(Saccharomyces, Candida, Kluyveromyces, etc.) have

been found to contain killer strains. These strains

secrete a protein that kills other strains of the same

species but not of unrelated species. The toxin binds

to a receptor on the wall of susceptible cells, then passes

to the membrane where it causes leakage of H+, and

the cells die owing to loss of transmembrane poten-

tial and disruption of amino acid uptake, K+balance,

etc. The toxins are stable only at low pH and are

thought to give a significant advantage to killer strains

over nonkiller strains of the same species in acidic

environments.

In Kluyveromyces lactisthe toxin is encoded by a

linear DNA plasmid, but in all other cases it is encoded

by dsRNA. Both the killer and nonkiller strains can

contain virus-like particles (VLPs) so this feature alone

does not correlate with killer activity. However, these

particles are found to be of two types: the dsRNA in

the “L type” encodes the virus coat protein, whereas

the dsRNA in the “M type” codes for the toxin. The M

dsRNA depends on the L dsRNA for the coat protein,

but the L dsRNA can occur alone with no effect on

the cells.

The molecular biology of the killer system in S. cere-

visiaehas been studied intensively and has shown

why the toxin producers are not affected by their own

toxin. They produce the toxin as a large precursor pro-

tein which undergoes changes during passage through

the secretory system to produce a protoxin. This

protoxin is finally cleaved at the cell membrane, to

release the active toxin but leave part of the molecule

in the membrane. This residual part seems to interact

with a toxin receptor, making the cell resistant to

active toxin in the external environment.

Similar dsRNA killer systems are found in the yeast

phase of Ustilago maydis(Basidiomycota) which causes

smut disease of maize. A killer system might also

occur in the take-all fungus Gaeumannomyces graminis,

because some dsRNA-containing strains can markedly

inhibit the growth of other strains at low pH. However

the mechanism in this case remains unknown.

FUNGAL GENETICS, MOLECULAR GENETICS, AND GENOMICS 173

Fig. 9.12Structures of two saponins that are pre-formed resistance compounds in plants: avenacin in roots of oats,
and α-tomatine in tomato. Pathogens with the appropriate enzymes can detoxify these compounds by cleaving some
of the terminal sugar residues.