Regulatory Function of b Polypeptides

After recognition of mating partners through a system based on pheromones, further sexual development occurs through a multiallelic b locus which determines whether a filamentous, pathogenic dikaryon is formed or not. For fusion of sporidia to occur, the nuclei must have two different b alleles (Table 8.1). Thirty-three different b alleles (bE 1 to 33 and bW 1 to 33) have been identified. This means that a system of recognition of nuclei exists by which any combination of 332 - 33 or 1056 different b alleles are active and 33 combinations of the same allele are inactive (Kämper et al., 1995). The clue to this system emerged from the cloning of the b locus. The cloning strategy for the b locus is based on the observation that the diploid strain a1a2 b1b2 strain develops fuzziness (limited filamentous growth) on artificial media containing charcoal. The introduction by transformation of the b allele in a diploid cell that is heterozygous for a but homozygous for b gives the "fuz," indicative of a mating reaction. The fuz will induce tumors if introduced in a corn plant; therefore, the fuz reaction is a valid test of successful mating. The fuz reaction allows the cloning of the b locus and determination of its nucleotide sequence.

Table 8.1

Mating Reaction of Sporidia of Ustilago maydis

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