Negative regulation of the proteolytic activation of a developmental transcription factor in Bacillus subtilis

The sporulation transcription factor σ(K) of Bacillus subtilis is controlled by a signal transduction pathway that operates at the level of the proteolytic processing of the inactive precursor protein pro-σ(K). The conversion of pro-σ(K) to σ(K) requires the putative processing enzyme SpoIVFB and is governed by the regulatory proteins SpoIVFA and BofA. We engineered vegetative cells to carry out processing of pro-σ(K) by inducing the synthesis of the proprotein, a modified form of the putative processing enzyme, and its two regulators during growth. The results showed that (i) modified SpoIVFB was the only sporulation protein necessary to achieve processing of pro-σ(K); (ii) SpoIVFA stimulated processing, apparently by protecting the processing enzyme from degradation; (iii) BofA inhibited processing in a manner that did not involve degradation of SpoIVFB; and (iv) the inhibition of SpoIVFB by BofA was dependent on SpoIVFA. We conclude that BofA and SpoIVFA act synergistically and are the only two sporulation proteins needed to inhibit the function of SpoIVFB. Our results are consistent with the idea that activation of pro-σ(K) occurs by a reversal of the BofA/SpoIVFA-mediated inhibition of the processing enzyme.