The b mating-type locus of Ustilago maydis contains variable and constant regions.

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Abstract
The b locus of the phytopathogenic fungus Ustilago maydis encodes a multiallelic recognition function that controls the ability of the fungus to form a dikaryon and complete the sexual stage of the life cycle. The b locus has at least 25 alleles and any combination of two different alleles, brought together by mating between haploid cells, allows the fungus to cause disease and undergo sexual development within the plant. An open reading frame of 410 amino acids has been shown to specify a polypeptide responsible for the activity of the b1 allele, and comparisons of the predicted amino acid sequences for 6 b alleles allowed identification of variable and constant regions within the coding region of the gene. Haploid strains carrying a null mutation at the b locus, created by gene disruption, are viable but fail to interact with formerly compatible strains to give an infectious dikaryon. Analysis of mutants carrying a null allele indicated that the products of different alleles of the b locus combine to form a new regulatory activity and that this activity directly or indirectly turns on the pathway leading to sexual development and pathogenesis.