Construction of chimeric alleles with altered specificity at the b incompatibility locus of Ustilago maydis

Multiallelic incompatibility systems found in many fungi and plants function to limit inbreeding by mediating self versus nonself recognition. The plant pathogenic fungus Ustilago maydis has a locus called b that governs incompatibility. Two multiallelic genes, bE and bW, are present at the b locus. Fusion of haploid strains carrying different alleles at bE and bW establishes an infectious dikaryon capable of pathogenesis on maize (Zea mays). Cells carrying a single type of b locus, whether haploid or dikaryotic, are nonpathogenic. To identify sequences within the bE gene that determine allelic specificity, targeted gene replacement was employed to produce a series of chimeras between the b1E and b2E alleles. Incompatibility tests with strains carrying the chimeric alleles identified a 30- to 48-amino acid region responsible for specificity. Suprisingly, the chimeras with recombination points within this region had a specificity different from both parent alleles. Overall, these results define an important domain in bE involved in self versus nonself recognition.