| Summary: | 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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