Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
269
Fig10.6 Mating type switch in Saccharomyces
cerevisiae. A germinating a-type ascospore
denoted by its white nucleus produces a bud
which has the same mating type. Ash1mRNA is
selectively translocatedinto the bud (a), and its
protein Ash1p is expressed in the daughter
nucleus (b), preventing it from switching its
mating type. However, the mother cell is now
competent to switch its mating type to a
because it has divided once before (c); when it
undergoes its second division (d,e), it produces
an a-type daughter cell.The
first-formed daughter cell cannot switch its
mating type because it has no previous history
of cell division.Consequently it produces a
daughter of a-type. Mating (f) occurs between
one a-andonea-type cell apiece, and conjugation
results in two diploid cells which can
reproduce by further budding (g) or by meiosis
and ascus formation. Based on Haber (1998).
HO endonuclease, the gene of which is only
expressed in mother cells which have divided
at least once (Nasmyth, 1983). Expression of the
HO endonuclease gene is controlled by a series of
repressor proteins, and one of them Ash1p is
confined to the daughter cell upon division. This
is due to the selective transport of its mRNA
molecule into the bud prior to cytokinesis (Long
et al., 1997).
Hicks et al. (1977) proposed the cassette model
to account for the mating type switch, and this
has been confirmed by subsequent experimentation.
In addition to the mating type locus which
is active in a given haploid cell, each haploid
genome possesses two further complete copies,
one to the left of the active locus which usually
contains the a allele (i.e. HMLa) and the other to
the right, encoding a (i.e. HMRa). These genes are
silenced, i.e. they are not transcribed because
their DNA is coated by histones and other proteins
encoded and regulated by numerous other
genes. Silencing is determined by the location
of these gene copies in the proximity of silencing
sequences (Haber, 1998). The mating type switch
is perfomed when the HO endonuclease cleaves
the currently active locus at the Y/Z boundary
(see Fig. 10.5), followed by the digestion of one
of the two DNA strands of the Z region by an
exonuclease. A new sequence is then copied
into that gap from either of the two silent genes,
using the one-stranded Z region as a template.
The integrity of the silent gene which acts as