DUAL SETS OF CHIMERIC ALLELES IDENTIFY SPECIFICITY SEQUENCES FOR THE BE AND BW MATING AND PATHOGENICITY GENES OF USTILAGO-MAYDIS

The b mating-type locus of the fungal plant pathogen Ustilago maydis e ncodes two multiallelic gene products, bE and bW, that control the for mation and maintenance of the infectious cell type, Dimerization via t he N-terminal regions of bF and bW proteins encoded by alleles of diff erent specificities establishes a homeodomain-containing transcription factor, The bE and bW products encoded by alleies of like specificiti es fail to dimerize, We constructed sets of chimeric alleles for the b E1 and bE2 genes and for the bW1 and bW2 genes to identify sequences t hat control specificity, The mating behavior of strains carrying chime ric alleles identified three classes of specificity: b2 (class I), spe cificity different from either parental type (class II), and b1 (class III). Crosses between strains carrying bE and bW chimeric alleles ide ntified two short blocks of amino acids that influence specificity and that are located in the N-terminal variable regions of the b proteins , Comparisons of pairs of chimeric alleles encoding polypeptides diffe ring in specificity and differing at single amino acid positions ident ified 16 codon positions that influence the interaction between bE and bW, Fifteen of these positions lie within the blocks of amino acids i dentified by crosses between the strains carrying chimeric alleles, Ov erall, this work provides insight into the organization of the regions that control recognition.