PHEROMONES TRIGGER FILAMENTOUS GROWTH IN USTILAGO-MAYDIS

Cell recognition and mating in the smut fungus Ustilago maydis have be en proposed to involve specific pheromones and pheromone receptors. Th e respective structural genes are located in the a mating type locus t hat exists in the alleles a1 and a2. We demonstrate that binding of ph eromone to the receptor can induce a morphological switch from yeast-l ike to filamentous growth in certain strains. Using this as biological assay we were able to purify both the the a1 and the a2 pheromone. Th e structure of the secreted pheromones was determined to be 13 amino a cids for a1 and nine amino acids for a2. Both pheromones are post-tran slationally modified by farnesylation and carboxyl methyl esterificati on of the C-terminal cysteine. An unmodified a1 peptide exhibits drama tically reduced activity. The pheromone alone is able to induce charac teristic conjugation tubes in cells of opposite mating type and confer s mating competence; evencells of the same mating type undergo fusion. We discuss the role of pheromones in initiating filamentous growth an d pathogenic development.