Industrial yeasts, including a sake yeast Kyokai no. 7 (1(7), are generally unable to sporulate. In K7 (Saccharomyces cerevisiae) cells, IME1 transcription was not induced under sporulation conditions, and K7 cells partially restored sporulation ability when transformed with a multicopy plasmid bearing IME1. However, the mechanisms of sporulation incompetence in industrial yeasts are poorly understood. We demonstrated that the deletion of the Cl cyclin CLN3, a key activator of the cell cycle, allows 1(7 cells to induce IME1 transcription and sporulate under sporulation conditions. In 1(7 cells, CLN3 mRNA and protein were not down-regulated despite sporulation conditions. Moreover, using a two-hybrid assay, we found that Ime1-Ume6 interaction was promoted in Cln3-deficient 1(7 cells. Thus, Cln3 is involved in the mechanism underlying sporulation incompetence by inhibiting IME1 transcription and the Ime1-Ume6 interaction. Based on these findings, we hypothesize that the absence of transmission of nutrient starvation signals to CLN3 leads to sporulation incompetence in 1(7 cells. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.