In the yeast Saccharomyces cerevisiae cell cycle-regulated SW15 transcription is essential for ensuring that mother and not daughter cells switch mating type. We have identified a 55-bp promoter sequence that appears to be responsible for restricting transcription to the late S, G2, and M phases of the cell cycle. Two proteins, MCM1, a transcription factor described previously, and SFF (SWI five factor, a newly identified factor) bind this sequence in vitro. MCM1 binds the DNA tightly on its own, but SFF will only bind as part of a ternary complex with MCM1. We observe a strong correlation between the ability of mutated SWI5 promoter sequences to form a ternary MCM1-SFF-containing complex in vitro and to activate transcription in vivo, which suggests that efficient transcription requires that both proteins bind DNA. Through its interactions with cell type-specific coactivators and corepressors, MCM1 controls cell type-specific expression of pheromone and receptor genes. By analogy, we propose that SFF enables MCM1 to function as a part of a cell cycle-regulated transcription complex.