Dual Sets of Chimeric Alleles Identify Specificity Sequences for the bE and bW Mating and Pathogenicity Genes of Ustilago maydis

Abstract

The b mating-type locus of the fungal plant pathogen Ustilago maydis encodes two multiallelic gene products, bE and bW, that control the formation and maintenance of the infectious cell type. Dimerization via the N-terminal regions of bE and bW proteins encoded by alleles of different specificities establishes a homeodomain-containing transcription factor. The bE and bW products encoded by alleles of like specificities fail to dimerize. We constructed sets of chimeric alleles for the bE1 and bE2 genes and for the bW1 and bW2 genes to identify sequences that control specificity. The mating behavior of strains carrying chimeric alleles identified three classes of specificity: b2 (class I), specificity different from either parental type (class II), and b1 (class III). Crosses between strains carrying bE and bW chimeric alleles identified two short blocks of amino acids that influence specificity and that are located in the N-terminal variable regions of the b proteins. Comparisons of pairs of chimeric alleles encoding polypeptides differing in specificity and differing at single amino acid positions identified 16 codon positions that influence the interaction between bE and bW. Fifteen of these positions lie within the blocks of amino acids identified by crosses between the strains carrying chimeric alleles. Overall, this work provides insight into the organization of the regions that control recognition.Recognition mediated by protein-protein interactions plays a fundamental role in many biological processes. Well-characterized examples include antibody-antigen interactions (8, 9, 23), ligand-receptor binding (22,35), and the establishment and maintenance of tissue integrity by cadherins (19). The proteins involved in sexual reproduction and incompatibility in fungi provide relatively simple examples of determinants of self versus nonself recognition. In this paper, we describe a molecular genetic approach to identify the determinants of recognition for the proteins encoded by the b mating-type locus of the fungal corn pathogen Ustilago maydis.U. maydis is commonly found in nature as black diploid teliospores on infected corn plants (6). The teliospores germinate, and meiosis occurs to produce haploid, yeast-like progeny. Nonself recognition between compatible haploid mating partners is a prerequisite to the establishment of an infectious, dikaryotic cell type, and the genes at the a and b mating-type loci are considered pathogenicity factors (reviewed in references 2 and 18). The a locus, with alternate specificities a1 and a2, encodes pheromones and pheromone receptors and controls recognition of mating partners at the level of cell fusion (3,11,31). The b locus controls the formation and maintenance of the infectious cell type after cell fusion has occurred. If the cells participating in mating have different specificities (nonself) at the b locus, a vigorous, straight dikaryotic filament is formed and this cell type will be infectious. In contrast, mating partners that carry b sequences of like specificities ...

The signature of balancing selection: Fungal mating compatibility gene evolution

“…Linkage to deleterious alleles seems an unlikely explanation in C. cinereus because recombination is not suppressed in noncoding regions within or flanking the A locus (17). Constraint on the total number of alleles could occur, although available evidence suggests that only a few amino acid substitutions may suffice to determine a new mating specificity (31,32). The suggestion that older lineages allow broader recognition capabilities and thus are preferentially retained through speciation events (4) is intriguing and could best be explored in a fungal system where site-directed mutations in compatibility genes can be made.…”

Sex in smut fungi: Structure, function and evolution of mating-type complexes

“…These two proteins can form a heterodimeric complex, but only when the proteins are derived from different alleles (Gillissen et al, 1992;Kämper et al, 1995), a situation which naturally occurs after mating of two compatible partners. The current model proposes that dimerization is achieved via a limited number of hydrophobic and polar interactions within the variable N-terminal regions of the bE and bW proteins (Kämper et al, 1995;Yee and Kronstad, 1998). This heterodimerization to form active transcription factors is the general principle for interaction between the b proteins of smut fungi.…”

Life History and Developmental Processes in the BasidiomyceteCoprinus cinereus

“…The failure to obtain this kind of self-compatible mutants indicates how specifically, precisely, and subtly the hypervariable N-terminal recognition regions work. Data from Ustilago (212,525,526) and yeast (175) suggest that there are many different contact points in the hypervariable N termini of HD1 and HD2 proteins that, depending on the particular protein combination, may contribute negatively or positively to the interaction. Therefore, it is thought that the summation of all contacts will decide upon dimerization; between incompatible proteins, this sum results in aversion, while between compatible proteins, it results in attraction.…”