Sporulation-specific sigma factor sigma 29 of Bacillus subtilis is synthesized from a precursor protein, P31.

Evidence is presented that a sporulation-essential sigma factor of Bacillus subtilis, sigma 29, is synthesized as an inactive precursor (P31) and that its activation occurs by a developmentally regulated cleavage of 29 amino acids from the P31 amino terminus. A pulse-chase experiment demonstrated that sigma 29 was derived from a preexisting protein, with appearance of radioactively labeled sigma 29 paralleling the disappearance of labeled P31. The disappearance of pulse-labeled P31 did not occur when the experiment was done with a B. subtilis strain carrying a mutation in a locus (spoIIE) required for sigma 29, but not P31, synthesis. Microsequencing of sigma 29 protein revealed that its amino terminus originates at amino acid 30 of the P31 amino acid sequence. In order to test whether a proteolytic event alone could activate P31 to a protein with sigma 29-like properties, a fusion protein (P31*) containing most of P31 was overproduced in Escherichia coli and converted in vitro into a protein with the electrophoretic mobility of sigma 29 by limited treatment with Staphylococcus aureus V8 protease. Protease-treated P31*, but not untreated P31*, was capable of directing B. subtilis core RNA polymerase to specifically initiate RNA synthesis at a sigma 29-recognized promoter in vitro.