Characterization and molecular cloning of conjugation-regulating sex pheromones in homothallic Closterium.

Conjugation-regulating pheromones were analyzed in homothallic Closterium for the first time. Members of the Closterium peracerosum-strigosum-littorale complex are unicellular charophycean algae in which there are two modes of zygospore formation: heterothallism and homothallism. A homothallic strain of Closterium (designation, kodama20) forms selfing zygospores via the conjugation of two sister gametangial cells derived from one vegetative cell. Conjugation-promoting and -suppressing activities, against cells at very low (1 x 10(2) cells ml(-1)) and normal (1 x 10(4) cells ml(-1)) cell density, respectively, were detected in the medium in which cells of a normal density had been cultured. Pheromone activities decreased to 20% after incubation at 60 °C for 10  min. The release and action of the pheromones was dependent on light. The culture medium was subjected to gel filtration, and both active substances had an apparent molecular mass of 17  kDa; this was similar to that previously reported for the heterothallic sex-specific pheromone protoplast-release-inducing protein (PR-IP) Inducer. cDNAs encoding the orthologs of PR-IP Inducer were isolated from the homothallic strain. Recombinant PR-IP Inducers produced by yeast cells showed conjugation-promoting activity. These results indicate that conjugation of the homothallic strain is regulated by an ortholog of a heterothallic sex-specific pheromone.