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

Summary Sporulation in Bacillus subtilis involves the formation of a polar septum, which divides the sporangium into a mother cell and a forespore. The σ E factor, which is encoded within the spoIIG operon, is a cell‐specific regulatory protein that directs gene transcription in the mother cell. σ E is synthesized as an inactive proprotein pro‐ σ E , which is converted to the mature factor by the putative processing enzyme SpoIIGA. Processing of pro‐ σ E does not commence until after asymmetric division when σ E 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 σ E is confined to the mother cell is not understood. Previous work favoured a model in which pro‐ σ E localizes to the mother cell face of the polar septum, such that σ E would be selectively released into mother cell cytoplasm. Based on the use of green fluorescent protein (GFP) fusions, we now report that pro‐ σ 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 spoIIG operon in the mother cell and degradation of σ E in the forespore contribute to the selective accumulation of σ E in the mother cell. Persistent transcription of spoIIG after polar septation also contributes to the proper timing of pro‐ σ E processing.