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Non-self recognition in filamentous fungi : the het-c mediated vegetative incompatibility in neurospora crassa Wu, Jennifer Donglan

Abstract

In Neurospora crassa, at least eleven het loci including the mating-type locus have been identified that regulate non-self recognition during vegetative growth. One of the genetically and molecularly characterized loci, het-c, was shown to encode three alleles het-cOR, het-cPA and het-cGR. The three het-c alleles encode similar polypeptides with a 34 - 48 amino acid polymorphic region that controls allelic specificity. In an effort to understand the biological role of the het-c locus in filamentous fungi, the sequences of het-c specificity region from species within Neurospora and its related genera were analyzed. The het-c locus exhibited a trans-specific mode of evolution (an allele from one species is more closely related to an allele from another species or genus than to other alleles from the same species) based on a phylogenetic analysis of DNA sequences and an increased frequency and number of nonsynonymous nucleotide substitutions in the polymorphic domain. This study suggested that the het-c locus is under balancing selection for the function of mediating non-self recognition. The polymorphic region in the peptides encoded by the three het-c alleles was dissimilar by both amino acid sequences and the pattern of deletion or insertion. To differentiate whether the composition of amino acid sequences or the pattern of insertion/deletion is the critical determinant for het-c allelic specificity, chimeric constructs of naturally occurring and artificially generated het-c alleles were introduced into N. crassa strains with alternative het-c specificities. Incompatibility of the transformants was assayed by occurrence of dead hyphal compartments, growth rate and colony morphology. This study suggested that spatial characteristics as affected by the pattern and size of the deletion/insertion within the specificity domain were the primary determinant for het-c allelic specificity. Immunoprecipitation studies indicated that non-self recognition is mediated by HET-c heteromeric complex formation during vegetative incompatibility, and that differences in the specificity domain affect the capacity to make heterocomplexes versus homocomplexes.

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