SpoIVB and CtpB are both forespore signals in the activation of the sporulation transcription factor σK in Bacillus subtilis

The proteolytic activation of the mother cell transcription factor pro-sigma(K) is controlled by a signal transduction pathway during sporulation in the bacterium Bacillus subtilis. The pro-sigma(K) processing enzyme SpoIVFB, a membrane-embedded metalloprotease, is held inactive by two other integral membrane proteins, SpoIVFA and BofA, in the mother cell membrane that surrounds the forespore. Two signaling serine proteases, SpoIVB and CtpB, trigger pro-sigma(K) processing by cleaving the regulatory protein SpoIVFA. The SpoIVB signal is absolutely required to activate pro-sigma(K) processing and is derived from the forespore compartment. CtpB is necessary for the proper timing of sigma(K) activation and was thought to be a mother cell signal. Here, we show that the ctpB gene is expressed in both the mother cell and forespore compartments but that synthesis in the forespore under the control of sigma(G) is both necessary and sufficient for the proper timing of pro-sigma(K) processing. We further show that SpoIVB cleaves CtpB in vitro and in vivo but that this cleavage does not appear to be necessary for CtpB activation. Thus, both signaling proteins are made in the forespore and independently target the same regulatory protein.