The 'pro' sequence of the sporulation-specific σ transcription factor σ(E) directs it to the mother cell side of the sporulation septum

σ(E), a mother cell-specific transcription factor of sporulating Bacillus subtilis, is derived from an inactive precursor protein (pro-σ(E)). Activation of σ(E) occurs when a sporulation-specific protease (SpoIIGA) cleaves 27 amino acids from the pro-σ(E) amino terminus. This reaction is believed to take place at the mother cell-forespore septum. Using a chimera of pro-σ(E) and green fluorescent protein (GFP) to visualize the intracellular location of pro-σ(E) by fluorescence microscopy, and lysozyme treatment to separate the mother cell and forespore compartments, we determined that the pro-σ(E)::GFP signal, localized to the forespore septum prior to lysozyme treatment, is restricted to the mother cell compartment after treatment. Thus, pro-σ(E)::GFP had been sequestered to the mother cell side of the septum. This segregation of pro-σ(E)::GFP, and presumably pro- σ(E), to the mother cell is likely to be the reason why σ(E) activity is restricted to that compartment.