Structural requirements for activity of the pheromones of Ustilago hordei

Ustilago hordei, the cause of barley-covered smut, initiates mating with pheromones. Gene sequence analysis suggested that these pheromones, Uhmfa1 and Uhmfa2, would be farnesylated peptides. Although isolation of mating-type-specific activity was rarely possible, chromatographic separations of culture supernatants yielded fractions that stimulated or inhibited mating. Based on predicted amino acid sequences and mass spectra of stimulating fractions, a series of pheromone analogs were synthesized and their activities were determined. Underivatized Uhmfa1 (PGKSGSGLGYSTC) or Uhmfa2 (EGKGEPAPYC) peptides were inactive, while peptides that were farnesylated and/or methyl esterified specifically induced conjugation tubes by cells of the opposite mating type. Uhmfa1 truncated from the amino terminus beyond the lysine lost activity, while truncated Uhmfa2 remained partially active. In mating bioassays, a pheromone concentration-dependent default mating response was observed. In competition studies, shorter Uhmfa1 peptides lacking pheromone activity inhibited activity of full-length peptides most effectively when both had the same functional groups.