SigE Commitment to sporulation and engulfment
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-σE in the mother cell. Synthesized during earlier stages in sporulation, pro-σE resides at the polar septum where it will be activated after formation of the septum is complete 102. A σF-dependent signal from the forespore in the form of SpoIIR instigates proteolytic cleavage of pro-σE by SpoIIGA 79, 80. Recently, it was shown that this process is coupled to fatty acid synthesis 102. Subsequently, σE 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 σE 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-σE in the forespore furthermore ensures confinement of σE activity to the mother cell 82, 104.
The σE 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 σE/σK co-dependency. These seem to form a specific group of genes that represent an overlap between the σE and σK 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 σE. 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-σK are also part of the σE regulon 100, 103, 107. Timing and levels of transcription of σE-dependent genes are fine-tuned by the mother cell-specific secondary regulators SpoIIID and GerR.