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Note: + = Successful mating reaction results in formation of a dikaryotic mycelium (fuz reaction). The a locus is biallelic and the b locus is multiallelic; however, only three are shown.

Note: + = Successful mating reaction results in formation of a dikaryotic mycelium (fuz reaction). The a locus is biallelic and the b locus is multiallelic; however, only three are shown.

The b locus contains a pair of divergently transcribed genes, bE (east) and bW (west), separated by a spacer region (Kämper et al., 1995). From the DNA sequence information, b polypeptides were predicted to have a constant and a variable domain and a homeodomain motif. Homeodomain proteins, first identified as products of Drosophila genes with an important role in embryonic development, bind to DNA and function as transcription factors regulating development. A yeast two-hybrid system (Chapter 6) was used to demonstrate that bE and bW proteins can associate into a dimer but only if they are derived from different alleles. It is postulated that only the pair-wise combinations of specific bE and bW that originate from different alleles of the locus interact through complementary hydrophobic sequences to form heterodimers (Kämper et al., 1995; Kahmann and Bölker, 1996). There is no interaction between proteins if they are encoded by the same mating type—self-nonself recognition is at the level of protein dimerization (Figure 8.8). Thus, no single strain is self-fertile. It is expected that we shall learn to what regulatory DNA sequences the combination of polypeptide encoded by one bE and one bW gene bind, what these transcripts code for

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