In fission yeast, meiosis is initiated by transcriptional activation of the mei3+ gene, under the combined influence of the four mating-type genes. The product of the mei3+ gene acts as a critical meiotic inducer by binding non-covalently to a newly identified protein kinase encoded by the ran1+ gene and inhibiting its enzymatic activity. Inactivation of the ran1+ protein kinase is both necessary and sufficient to divert a vegetative cell from mitotic division to meiotic differentiation.