An investigation into the compartmentalization of the sporulation transcription factor σE 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 sigma(E) factor, which is encoded within the spollG operon, is a cell-specific regulatory protein that directs gene transcription in the mother cell. sigma(E) is synthesized as an inactive proprotein pro-sigma(E), which is converted to the mature factor by the putative processing enzyme SpollGA. Processing of pro-sigma(E) does not commence until after asymmetric division when sigma(E) is largely confined to the mother cell. Processing depends on the signalling protein SpollR, which delays proteolysis until after polar septation, but the mechanism by which sigma(E) is confined to the mother cell is not understood. Previous work favoured a model in which pro-sigma(E) localizes to the mother cell face of the polar septum, such that sigma(E) would be selectively released into mother cell cytoplasm. Based on the use of green fluorescent protein (GFP) fusions, we now report that pro-sigma(E) 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 spollG operon in the mother cell and degradation of sigma(E) in the forespore contribute to the selective accumulation of sigma(E) in the mother cell. Persistent transcription of spollG after polar septation also contributes to the proper timing of pro-sigma(E) processing.