Growth, Differentiation and Sexuality

362 L.A. Casselton and M.P. Challen
MATα cells and their near-clonal distribution in
the wild.
II. Molecular Analysis
of Mating Type Genes
The organisation of the mating type loci of U. maydis,
C. cinereus, S. commune and C. neoformans are
illustrated in Figs. 17.4, 17.5 and 17.6.
A. Tetrapolar Species
1. The b and A Genes Encode
a Transcription Factor
The genes at the b and A loci of U. maydis and C.
cinereus encode two dissimilar proteins, each characterised
by a homeodomain DNA-binding motif
(Fig. 17.4). The genes and proteins belong to two
subfamilies that have been termed HD1 and HD2
on the basis of conserved but distinctly different
homeodomain sequences. These genes/proteins
Fig. 17.4.A–C Organisation of the homeodomain transcription
factor genes at the A b and B A mating type loci of
Ustilago maydis and Coprinus cinereus. C Hypothetical heterodimer
that results from a compatible protein interaction
on mating. The HD1 and HD2 homeodomain-encoding sequences
within the genes are represented by open and closed
boxes respectively. Different alleles of the paired genes are
represented by different fill motifs. Horizontal arrows in-
are, in fact, the homologues of the a1 andα2
mating type genes/proteins of S. cerevisiae (Kües
and Casselton 1992). In S. cerevisiae, the two
classes of proteins are encoded by genes in different
haploid cells, and it is only after cell fusion
that they are present together in the resulting
diploid cell, and they then heterodimerise to
form a diploid cell-specific transcription factor
(Herskowitz 1988). In the basidiomycetes, by
contrast, every cell produces an HD1 and an HD2
protein. The b locus of U. maydis spans some 4 kb
and contains a single pair of genes, bW and bE,
that are divergently transcribed from a common
promoter sequence (Gillissen et al. 1992). The HD1
gene (bE) corresponds to α2 andtheHD2 gene
(bW) corresponds to a1 (Gillissen et al. 1992).
Like a1 andα2, a compatible cell fusion leads
to heterodimerisation between an HD1 and an
HD2 protein, to generate a transcription factor
complex unique to mated cells – in this case,
the dikaryon. Self-mating is prevented because
the genes found together at the b locus (e.g.
bW1-bE1) encode proteins that are unable to
heterodimerise. Heterodimerisation is possible
dicate direction of transcription whereas diagonal arrows
indicate compatible gene combinations that lead to heterodimerisation
of the corresponding proteins. The A locus
is flanked by the mip gene encoding an intermediate mitochondrial
endopeptidase that is highly conserved in position
in other homobasidiomycete species (Kües et al. 2001;
James et al. 2004a)