Circular chromosomes have frequently been observed in tumors of mesenchymal origin. In the fission yeast Schizosaccharomyces pombe, deletion of pot1+ results in rapid telomere loss, and the resulting survivors have circular chromosomes. Fission yeast has 2 bromodomain and extra-terminal (BET) proteins, Bdf1 and Bdf2; both are required for maintaining acetylated histones. Here, we found that bdf2, but not bdf1, was synthetically lethal with pot1. We also obtained a temperature-sensitive bdf2-ts mutant, which can grow at high temperatures but becomes camptothecin sensitive. This suggests that Bdf2 is defective at high temperatures. The cell cycle of the pot1 bdf2-ts mutant was delayed in the G2 and/or M phase at a semipermissive temperature. Furthermore, a temperature-sensitive mutant of mst1, which encodes histone acetyltransferase, showed a synthetic growth defect with a pot1 disruptant at a semipermissive temperature. Our results suggest that Bdf2 and Mst1 are required for the growth of cells with circular chromosomes.