Molecular genetics of mating recognition in basidiomycete fungi

Abstract

The recognition of compatible mating partners in the basidiomycete fungi requires the coordinated activities of two gene complexes defined as the mating-type genes. One complex encodes members of the homeobox family of transcription factors, which heterodimerize on mating to generate an active transcription regulator. The other complex encodes peptide pheromones and 7-transmembrane receptors that permit intercellular signalling. Remarkably, a single species may have many thousands of cross-compatible mating types because the mating-type genes are multiallelic. Different alleles of both sets of genes are necessary for mating compatibility, and they trigger the initial stages of sexual development--the formation of a specialized filamentous mycelium termed the dikaryon, in which the haploid nuclei remain closely associated in each cell but do not fuse. Three species have been taken as models to describe the molecular structure and organization of the mating-type loci and the genes sequestered within them: the pathogenic smut fungus Ustilago maydis and the mushrooms Coprinus cinereus and Schizophyllum commune. Topics addressed in this review are the roles of the mating-type gene products in regulating sexual development, the molecular basis for multiple mating types, and the molecular interactions that permit different allelic products of the mating type genes to be discriminated. Attention is drawn to the remarkable conservation in the mechanisms that regulate sexual development in basidiomycetes and unicellular ascomycete yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, a theme which is developed in the general conclusion to include the filamentous ascomycetes Neurospora crassa and Podospora anserina.

FIG. 1

Life cycle of C. cinereus.

FIG. 2

Roles of the A and B mating-type genes in regulating the formation and maintenance of the dikaryon of C. cinereus.

FIG. 3

Life cycle of the pathogenic smut fungus U. maydis.

FIG. 4

Molecular structure and organization of the b mating-type locus of U. maydis. (A) The multiallelic b locus contains two divergently transcribed genes, bE and bW, which encode polypeptides of 473 and 644 amino acids, respectively (36, 108). The sequence of the regions amino-terminal to the homeodomains of the two proteins and the intervening 260-bp promoter region is unique to each b specificity. The b1 and b2 loci are illustrated, and the variable DNA sequence specific to each is represented by the light grey and dark grey regions, respectively. Striated boxes indicate the homeodomain-encoding sequences. (B) Gene deletion experiments showed that the compatible gene combination that promotes b-regulated sexual development is a bE gene from one locus and a bW gene from the other (diagonal arrow). For b1 and b2, the compatible gene combinations are bE1 + bW2 and bE2 + bW1.

FIG. 5

Molecular structure and organization of the A mating-type locus of C. cinereus and S. commune. (A) The predicted archetypal A locus of C. cinereus contains three pairs of divergently transcribed multiallelic genes (a, b, and d). The genes in each pair encode two dissimilar proteins with distinctive homeodomain motifs designated HD1 and HD2. HD1 genes (a1, b1, and d1) encode proteins of ∼680 amino acids, and HD2 genes (a2, b2, and d2) encode proteins of ∼520 amino acids. Thick black lines indicate homologous DNA sequences flanking the locus and the common 7-kb sequence (the homologous hole) between the a and b genes which divides the genes into two complexes, α and β (65, 69). Thin lines indicate nonhomologous DNA sequences that do not permit recombination between genes. The a genes are represented by open boxes, the b genes are represented by grey boxes, and the d genes are represented by solid boxes. The homeodomain-encoding sequences are represented by striated boxes. (B) Organization of the A5 and A6 loci. The events that led to gene deletion may explain why d2-2 is not oriented as predicted. Diagonal arrows indicate the compatible HD1-HD2 gene combinations that promote A-regulated sexual development. The dashed line indicates that the region of nonhomology has not been fully defined. (C) The Aα locus of S. commune contains a single pair of divergently transcribed multiallelic HD1 and HD2 genes designated Z and Y, respectively, which encode proteins of 890 to 930 amino acids (122). The Aα1 locus contains only the Y gene (Y1), but the other eight versions of this locus are predicted to contain both genes. The diagonal arrow indicates a compatible gene combination that promotes A-regulated development.

FIG. 6

Predicted structural features of the HD1 and HD2 A mating-type proteins of C. cinereus, and the constitutively active protein encoded by an HD2-HD1 gene fusion. Diagonally striped boxes indicate α-helical domains, solid boxes indicate the homeodomains, and grey boxes indicate a negatively charged region that is a putative activation domain. N indicates the position of bipartite NLSs.

FIG. 7

Overview of the pheromone response pathway in S. cerevisiae. The pheromone binds to the extracellular loops of the receptor. The heterotrimeric G-protein is entirely membrane bound throughout the signalling process but is drawn free for simplicity. The Gα subunit is anchored to the cell membrane and interacts with the receptor through its carboxy-terminal domain and with the Gβγ subunit through its amino-terminal domain. Gγ is responsible for anchoring Gβγ to the cell membrane, whereas Gβ interacts with the Gα subunit (inactive state) and activates Ste20p (active state). Ste5p acts as a scaffold to anchor the members of the MAP kinase module. Ste12p binds to PREs in the promoters of a wide range of genes.

FIG. 8

Molecular structure and organization of mating-type loci encoding pheromones and pheromone receptors. (A) The a1 locus of U. maydis spans ∼4.5 kb and contains two genes, mfa1 (encoding the a1 pheromone) and pra1 (encoding the receptor). The a2 locus spans ∼8 kb and contains the corresponding genes, mfa2 (encoding the a2 pheromone) and pra2 (encoding the receptor). Two additional genes with unknown function, lga2 and rga2, reside in the a2 locus. Arrows indicate the directions of gene transcription, and thick lines indicate the homologous flanking regions bordering the mating-type-specific DNA sequences. (B) Each locus of S. commune spans ∼8 kb of allele-specific DNA sequence which contains four genes, three (small boxes) encoding pheromone precursors and one (large box) encoding a receptor. The Bα genes are shown as open boxes and the Bβ genes are shown as solid boxes to indicate that they belong to two functionally independent subfamilies. The Bα and Bβ loci may be separated by up to 3.5 map units (97). Dashed lines indicate that the borders of the homologous region have not yet been defined. (C) The B locus of C. cinereus spans ∼17 kb of locus-specific DNA sequence that contains three subfamilies of functionally independent genes represented by the open, grey, and solid boxes. Small boxes represent pheromone genes, and large boxes represent receptor genes. The B6 and B42 loci encode different alleles of the genes in subfamilies 2 and 3, as indicated by the upward or downward diagonals. The alleles of the genes in subfamily 1 are shared.

FIG. 9

Predicted amino acid sequences of pheromone precursors encoded by genes of the B mating-type loci of C. cinereus and S. commune. The amino acid sequences encoded by the six genes in the B6 locus of C. cinereus (Phb 1.1, Phb 1.2, Phb 2.1, Phb 2.2, Phb 3.1, and Phb 3.2) and the six genes in the Bα1 [Bap1(1), Bap1(2), and Bap1(3)] and Bβ1 [Bbp1(1), Bbp1(2), and Bbp1(3)] loci of S. commune have been aligned. Each sequence encodes a carboxy-terminal CaaX motif (CVIA/CVIS/CVCH/CVRG/CVVA) which acts as a signal for post-translational isoprenylation, and most encode a two-residue charged motif (ER/DR/EH/NH, indicated by arrows) that may be a signal for cleavage to generate the mature peptide pheromone. The two pheromones [Bap1(1) and Bbp1(2)] which lack this doubly charged motif have a comparable motif (SR/NR) at a similar position.