SporeWeb

SigE Commitment to sporulation and engulfment (Stages II - III)

SigE during commitment to sporulation

Further progress into the sporulation process, characterized by engulfment of the forespore, initiation of spore coat assembly and prevention of asymmetric division in the mother cell, is brought about by the activation of pro-σ^E in the mother cell. Synthesized during earlier stages in sporulation, pro-σ^E resides at the polar septum where it will be activated after formation of the septum is complete ¹⁰². A σ^F-dependent signal from the forespore in the form of SpoIIR instigates proteolytic cleavage of pro-σ^E by SpoIIGA ^{79, 80}. Recently, it was shown that this process is coupled to fatty acid synthesis ¹⁰². Subsequently, σ^E is released from the septum into the cytoplasm of the mother cell where it initiates the transcription of a subset of mother cell-specific sporulation genes ^{81, 100, 103}. Containment of this action to the mother cell compartment is thought to be achieved by the cooperative effect of several events. Firstly, an increase in activated Spo0A~P levels in the mother cell compartment leads to elevated levels of the spoIIG transcript and σ^E and also prevents further transcription of spoIIG in the forespore . Inhibition of Spo0A-dependent gene transcription ¹⁰⁵ and possible degradation of already present Spo0A and pro-σ^E in the forespore furthermore ensures confinement of σ^E activity to the mother cell ^{82, 104}.

The σ^E regulon encompasses the largest of the sporulation regulons with 253 genes identified ¹⁰¹. Expression profiles of these genes can be divided into three subgroups, depending on the timing and duration of expression ¹⁰⁰. Some of the later expressed genes also depend on secondary mother cell-specific regulator SpoIIID, whereas others have σ^E/σ^K co-dependency. These seem to form a specific group of genes that represent an overlap between the σ^E and σ^K stages of sporulation ⁹⁵¹⁰⁰. The spoIID, spoIIM and spoIIP genes, essential for correct completion of the engulfment process, as well as early spore coat assembly genes and spore cortex synthesis genes are transcribed under the control of σ^E. Furthermore, several genes important for gene regulation and progression into further stages of the sporulation process such as spoIIID, sigK and genes required for processing and activation of pro-σ^K are also part of the σ^E regulon ^{100, 103, 107}. Timing and levels of transcription of σ^E-dependent genes are fine-tuned by the mother cell-specific secondary regulators SpoIIID and GerR.