Cloning and characterization of a cDNA encoding a sexual cell division-inducing pheromone from a unicellular green alga Closterium ehrenbergii (Chlorophyta)

A full-length cDNA encoding a sexual cell division-inducing pheromone (SCD-IP) was isolated from a heterothallic green alga, Closterium ehrenbergii Menegh. ex Ralfs, using the rapid amplification of cDNA ends-PCR method. The cDNA was 907 bp in length and coded for a protein of 209 amino acids with a deduced molecular mass of M-r 23,186. The predicted protein had a possible glycosylation site and a hydrophobic signal peptide for extracellular secretion. Transcripts of the SCD-IP gene were detected only in mating type minus (mt(-)) cells and remarkably increased when mt(-) cells were cocultured with mating type plus (mt(-)) cells. By contrast, the gene for SCD-IP was detected in both mating type cells as multicopy genes by Southern blot analysis. Continuous dark conditions and supplementation of a nitrogen source to the culture medium suppressed SCD-IP expression. The deduced amino acid sequences encoded by genes for SCD-IP and protoplast-release-inducing protein (PR-IP) Inducer, a sex pheromone isolated from another Closterium species, appeared to share significant homology, suggesting that these sex pheromones were diversified from a single ancestral Closterium pheromone.