We have previously demonstrated thatStreptococcus pneumoniae signal peptidase (SPase) I catalyzes a self-cleavage to result in a truncated product, SPase37–204 [Peng, ., Wang, L., Moomaw, J., Peery, ., Sun, ., Johnson, ., Lu, J., Treadway, P., Skatrud, . & Wang, . (2001)J. , 621–627]. In this study, we investigated the effect of phospholipid onin vitro self-cleavage ofS.