An investigation into the compartmentalization of the sporulation transcription factor sigmaE in Bacillus subtilis

Sporulation in Bacillus subtilis involves the formation of a polar septum, which divides the sporangium into a mother cell and a forespore. The sigmaE factor, which is encoded within the spoIIG operon, is a cell-specific regulatory protein that directs gene transcription in the mother cell. SigmaE is synthesized as an inactive proprotein pro-sigmaE, which is converted to the mature factor by the putative processing enzyme SpoIIGA. Processing of pro-sigmaE does not commence until after asymmetric division when sigmaE is largely confined to the mother cell. Processing depends on the signalling protein SpoIIR, which delays proteolysis until after polar septation, but the mechanism by which sigmaE is confined to the mother cell is not understood. Previous work favoured a model in which pro-sigmaE localizes to the mother cell face of the polar septum, such that sigmaE would be selectively released into mother cell cytoplasm. Based on the use of green fluorescent protein (GFP) fusions, we now report that pro-sigmaE is distributed approximately uniformly along all membrane surfaces and is not confined to the mother- cell face of the septum. Rather, our results are consistent with a model in which preferential and persistent transcription of the spoIIG operon in the mother cell and degradation of sigmaE in the forespore contribute to the selective accumulation of sigmaE in the mother cell. Persistent transcription of spoIIG after polar septation also contributes to the proper timing of pro-sigmaE processing.