Schizosaccharomyces pombe ste11 encodes a member of the family of HMG-box proteins. Its transcript is induced in response to nitrogen starvation and a concomitant decrease of the intracellular cAMP level. Expression of ste11 is essential for induction of sexual development, and its ectopic expression stimulates uncontrolled mating and sporulation. Ste11 protein regulates positively transcription of the following genes required for sexual development: the mating type genes, matP and matM, and the mei2 gene, which is essential for commitment to meiosis. Ste11 protein synthesized in vitro binds specifically to a DNA fragment carrying a 10-base motif TTCTTTGTTY that is an essential cis-acting element for the induction of mei2 and is commonly seen in the upstream regions of the genes inducible by nitrogen starvation. These observations strongly suggest that Ste11 serves as a key transcription factor for sexual development.