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Specificity at the b mating type locus of ustilago maydis

University of British Columbia

The ability to discriminate between self and nonself allows Ustilago maydis to maintain genetic diversity through a multiallelic mating type system governed by the b locus. There are approximately 25 different alleles of the b locus and a given individual is incompatible with "self, but compatible with all "nonself mating types. The b locus consists of two divergently transcribed genes, designated bE and bW, and recognition of "self occurs because bE and bW proteins from the same allele do not dimerize. Conversely, bE and bW proteins encoded by two different b alleles do form dimers, and these dimers are active as homeodomain transcription factors, regulating the processes of sexual reproduction and pathogenicity. The N-terminal domains of both bE and bW have been shown to contain the determinants of dimerization specificity. A large set of closely spaced chimeric alleles was constructed for each of the interacting genes, one set of bEl-bE2 chimeras, and another of bWl-bW2 chimeras. The specificity of each chimeric allele was determined and they fell into three classes; bl (class I), b2 (class DI), and specificity different from either bl or b2 (class IT). The close spacing of the chimeric alleles allowed specificity changes at class boundaries to be attributed to single amino acid differences. An examination of substitutions affecting specificity revealed that 3 out of 8 involved an amino acid with an aromatic side chain (Tyr or Phe). To gain further insight into the interaction between bE and bW, strains carrying chimeric alleles of bE and bW were crossed in all pairwise combinations. The patterns of compatible and incompatible crosses identified intermolecular interactions between two pairs of positions on bE and bW. Site directed mutagenesis was employed to confirm the interaction between one of these pairs of positions. The results indicated that Val at position 90 of bEl and Tyr at position 74 of bWl interact, either directly or indirectly, to inhibit dimerization between bEl and bWl. These data provide evidence for an "inhibitory determinant" model, whereby specificity between bE and bW proteins is mediated by residues that prevent interaction between self combinations.

Thesis/Dissertation

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2009-06-02

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http://hdl.handle.net/2429/8651

Plant Science

University of British Columbia

UBCV

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