A trace amount of the pro-apoptotic factor human Bax was sufficient to kill hostEscherichia coli(Asoh, S., Nishimaki, K., Nanbu-Wakao, R., and Ohta, S., submitted). The region of Bax lethal toE. colicells was determined by introducing truncated humanbaxmutant genes. A peptide corresponding to amino acid residues 115 to 144 of Bax was the smallest peptide capable of inducing cell death ofE. coli.A truncatedbaxgene (Bax112-192) containing the region lethal toE. coliwas then introduced into a murine promyeloid cell line, FDC-P1. Constitutively expressed Bax112-192 induced apoptosis as judged by decrease of transfectants surviving and DNA fragmentation. These results indicate that Bax112-192 contains the region directly responsible for mammalian apoptosis as well as bacterial death. Flow cytometric analysis by FITC-Annexin V showed that the transfectant cells expressing Bax112-192 or native Bax became apoptotic even without external stimuli. The apoptotic population in the cells expressing Bax112-192 was not decreased by co-expression of Bcl-2 or Bcl-xL, while Bcl-2 or Bcl-xLsuppressed apoptosis in the cells expressing native Bax. Therefore, Bax induces apoptosis by its own activity without blocking the anti-apoptotic activity involved in Bcl-2 or Bcl-xL.