Bacillus Subtilis Pro-sigmaE Fusion Protein Localizes to the Forespore Septum and Fails to Be Processed when Synthesized in the Forespore

Endospore formation in Bacillus subtilis begins with an asymmetric cell division that partitions the bacterium into mother cell and forespore compartments. Mother cell-specific gene expression is initiated by sigmaE, a transcription factor that is active only in the mother cell but which existed as an inactive precursor (pro-sigmaE) in the predivisional cell. Activation of pro-sigmaE involves the removal of 27 amino acids from its amino terminus. A chimera of pro-sigmaE and the green fluorescent protein (GFP) was expressed from either the normal sigE promoter (P(spoIIG)), which places pro-sigmaE::GFP in both mother cell and forespore compartments, or the forespore-specific promoter (P(dacF)), which produces pro-sigmaE::GFP only in the forespore compartment. The pro-sigmaE::GFP expressed from P(spoIIG), but not P(dacF), was converted to a lower-molecular-weight form by a mechanism dependent on gene products (SpoIIGA and sigmaF) that are essential for normal pro-sigmaE processing. This finding is consistent with the pro-sigmaE processing reaction occurring only in the mother cell compartment. In processing-deficient cells, pro-sigmaE::GFP was found to accumulate at the septal membrane, a location where its processing apparatus would be susceptible to triggering from the adjoining forespore.