Inhibitory effect of PepO on SpeB production. Culture supernatants prepared from different growth phases in C medium were analyzed by Western blotting, as detailed in Materials and Methods, using the anti-SpeB antibody. Arrows indicate the 40-kDa zymogen form (proSpeB) and 28-kDa mature form of SpeB (mSpeB) Lane 1, 5448 wild type (WT); lane 2, 5448ΔpepO; lane 3, 5448ΔpepO::pepO; lane 4, 5448 ΔspeB. EL, early log phase (A600, ∼0.4); ML, mid-log phase (A600, ∼0.8); SP, stationary phase (A600, ∼1.2).