Growth, Differentiation and Sexuality

226 C. Schimek and J. Wöstemeyer
vegetative branching (Hill 1996). The same applies
to all observations concerning increase in
transcription rates, cellular concentrations of
rRNA and mRNA, protein synthesis and histone
acetylation (Horowitz and Russel 1974; Silver and
Horgen 1974; Groner et al. 1976; Horgen 1977;
Sutherland and Horgen 1977; Michalski 1978;
Horgen et al. 1983). Most of these effects are
unspecific, and reflect the expression of increasing
cellular activities in the course of morphological
and physiological changes during the sexual
process.
The synthesis of a number of proteins is
specifically and directly induced or enhanced by
antheridiol (Horton and Horgen 1985), one of
these possibly being cellulase (Groner et al. 1976;
cf. above). This specific pheromone-mediated
response has been investigated in greater detail.
Over a period of almost two decades, Brunt
and Silver as well as Riehl and Toft and their
co-workers have elucidated the induction and
participation of certain proteins in the antheridiolmediated
sexual response of Achlya. They found
that antheridiol-regulated proteins occur at
different cellular localizations, and are always
to be grouped among the minor polypeptides
(Brunt and Silver 1986a,b, 1987). Besides its
influence on protein synthesis, treatment with
antheridiol may also affect protein processing.
Evidence exists that a number of glycoproteins
become deglycosylated at the onset of the sexual
reaction in the male A. ambisexualis E87 (Brunt
and Silver 1986a), implicating regulation of
cellular recognition events by the pheromone.
A 85-kDa protein band detected both in the
nuclear and the cytoplasmic fraction (Brunt
and Silver 1986b) was later found to consist
partially of a previously identified (Silver et al.
1983) 85-kDa heat-shock protein (Brunt et al.
1990; Brunt and Silver 1991). Based on antibody
cross-reactions and sequence similarity, this
protein is classified as a hsp-90 protein (Brunt
et al. 1990).
A specific antheridiol receptor was identified
by Riehl et al. (1984) in the cytoplasm of male cells
only. As the binding activity could be recovered in
fractions displaying different sedimentation coefficients,
the existence of a multiprotein complex was
deduced and later confirmed. At least the hsp90
protein is an integral part of that complex (Brunt
et al. 1990).
Several other heat-shock proteins, e.g. three
hsp-70 proteins,a23-kDaanda56-kDaprotein(Sil-
ver et al. 1993; Brunt et al. 1998a,b) as well as other
hormone-binding and non-binding polypeptides
(Riehl et al. 1985; Brunt et al. 1998b) are associated
with the steroid receptor-hsp complex. They constitute
a multiprotein heterocomplex where some
of the participating components are not necessarily
presentallthetimeorinallexistingcomplexes.
At the transcriptional level, two similar but different
transcriptional populations exist for hsp70
and hsp90, and these are regulated differently (Silver
et al. 1993; Brunt and Silver 2004). In each case,
both transcript populations are regulated by antheridiol,
but one of them also reacts to an unrelated
stimulus, hsp70 to decreased glucose concentrations
(Silver et al. 1993) and hsp90 to increased
temperature (Brunt and Silver 2004). Transcript divergence
from a single cDNA clone was already
documented by Horton and Horgen (1989), an observation
probably based on similar regulation processes.
Consistent with earlier assumptions, all authors
today agree on the oomycete steroid receptor
organization and its regulation strongly resembling
animal steroid hormone systems (Riehl and Toft
1984; Riehl et al. 1985; Brunt et al. 1998b; Brunt and
Silver 2004). This viewpoint is further supported
by the identification of a diversity of putative transcription
factor response elements in the 5 ′ region
of the hsp90 genes. Among these are motifs already
known from animal steroid hormone response elements
(Brunt et al. 1998a).
The Phytophthora parasitica and P. infestans
mating type locus have been analysed in detail by
the group around Judelson. Using RAPD markers
to identify loci linked to the A1 and A2 phenotypes,
genetic and physical mapping of these loci was possible.
In both species, a bipolar mating system exists.
Mating types are determined by heterozygosity
in A1 (allele combination Aa) and by homozygosity
in A2 (aa) at the single mating type locus (Judelson
et al. 1995; Judelson 1996a). In P. infestans,
an unusual segregation pattern of the mating type
alleles prevails, showing a preference for two of
the four possible genotypes. Despite the chromosomes
bearing the A1 and A2 determinants being
genetically similar, a region of structural heterozygosity,
locus S1, flanks the mating type locus in
A1 isolates whereas it is absent in A2. Although S1
contains no obvious open reading frames, a function
of this sex chromosome-like region in the regulation
of allele segregation, DNA replication or
gene expression seems plausible (Judelson 1996b;
Randall et al. 2003). In Phytophthora parasitica,