gene causes the gradual displacement of wild-type
mitochondrial DNA (reviewed by Esser 1990; Bertrand
1995). The strains of Podosporathat exhibit the senes-
cence phenotype can be maintained indefinitely as
repeatedly subcultured young colonies, but they stop
growing, become senescent and die after they have
been grown continuously for about 25 days. It has long
been known that a non-nuclear “infective factor” is
involved, because nonsenescent strains (which never
undergo senescence) acquire the ability to senesce when
their hyphae anastomose with senescence-prone strains.
Moreover, mitochondria were implicated because the
onset of senescence could be postponed indefinitely
by growing strains in the presence of sublethal doses
of inhibitors of mitochondrial DNA synthesis or mito-
chondrial protein synthesis, but senescence occurred
when the inhibitors were removed. More recent
studies showed that DNA from strains that were
undergoing senescence could be transformed into pro-
toplasts of healthy strains, and the protoplast progeny
senesced immediately. The cause of this seems to be a
95 kb plasmid which normally exists as an integral part
of the mitochondrial DNA of healthy strains or of
juvenile (pre-senescent) cultures of senescent strains. But
as the senescent strains age this DNA is excised from
the mitochondrial genome, becomes a closed circular
molecule, and self-replicates, causing senescence.
Precisely how it does this is still in doubt, but the
plasmid shows DNA homology with an intron in
one of the mitochondrion genes – the gene that codes
for a subunit of cytochrome-c-oxidase, an enzyme
essential for normal function of the respiratory electron
transport chain. Senescent strains ofPodosporalack
cytochrome-c-oxidase activity, perhaps because the
plasmid inserts in the mitochondrial DNA, leading to
disruption of gene function or causing mitochondrial
gene rearrangements. Dysfunction of cytochrome-c-
oxidase or other components of electron transport
would be lethal for Podosporabecause this fungus seems
unable to grow anaerobically by fermenting sugars.
Plasmids and transposable elements
Plasmids usually are closed-circular molecules of DNA
with the ability to replicate autonomously in a cell.
However, they can also be linear DNA molecules if
the ends are “capped” (like chromosomes) to prevent
their degradation by nucleases. Plasmids or plasmid-like
DNAs have been found in several fungi. The most
notable example is the “two-micron” plasmid of S.
cerevisiae, so-called because of its 2μm length as seen
in electron micrographs. This plasmid is a closed
circular molecule of 6.3 kb, and it is unusual because
it is found in the nucleus, where it can be present in
up to 100 copies. It has no known function, but in the
past it was used to construct “vectors” for gene
cloning in yeast.
Most other plasmids of fungi are found in the
mitochondria. The best characterized are the linear
DNA plasmids of Neurospora crassaand N. intermedia.
They show a degree of base sequence homology to
the mitochondrial genome, suggesting that they are
defective, excised segments of the mitochondrial
genes. However, some other mitochondrial plasmids
of Neurosporaare closed circular molecules with little
or no homology to the mitochondrial genome. They
have a variable “unit” length of about 3–5 kb (in dif-
ferent cases) and the units can join head-to-tail to form
larger repeats. None of these fungal plasmids has any
known function, so they are not like bacterial plasmids
that code for antibiotic resistance, pathogenicity, or the
ability to degrade pesticides, etc.
Transposons (transposable elements) are short
regions of DNA that remain in the chromosome but
encode enzymes for their own replication. They pro-
duce RNA copies of themselves, and they encode the
enzyme, reverse transcriptase, which synthesizes
new copies of DNA from this RNA template, similar to
the action of retroviruses such as HIV. The new copies
of DNA can then insert at various points in the same
or other chromosomes, leading to alterations in gene
expression. Transposons seem to be rare in filamentous
fungi, but there are several types in S. cerevisiae. The
best studied of these are the chromosomal Ty elements,
present in about 30 copies in yeast cells. Oliver (1987)
described the known and possible roles of Ty elements
(Fig. 9.3). In addition to a role in altering gene expres-
sion, they could have significant effects on chromo-
somal rearrangements when the “delta sequences” on
the ends of these elements combine with one another.
These transposable elements seem to have no function,
except for self-perpetuation.
The mating-type genes of S. cerevisiaeare transposable
casettes, causing mating-type switching as discussed
in Chapter 5. However, mating-type switching cannot
occur in Neurospora crassa, because individual haploid
strains consist of only one mating type.Viruses and viral genesFungal viruses were first discovered in the 1960s,
associated with “La France” disease of the cultivated
mushroom Agaricus bisporus. (The name was coined by
British mushroom growers, reflecting the entente cordiale
that has long characterized Anglo-French relations!)
In this disease the fruitbodies are distorted and the
fruitbody yield is poor. Electron micrographs of both
the hyphae and the fruitbodies showed the presence
of many isometric virus-like particles (VLPs), assumed
to be the cause of the problem. VLPs were then