SigG Completion of engulfment and cortex systhesis
SigG during engulfment
The activation of σG in the forespore is tightly regulated by various control mechanisms for which the details are still largely unknown. σG activation is restricted to the forespore and prevented to occur in the mother cell by the combined action of the LonA protease and SpoIIAB 116, 136, 142. Even though SpoIIAB is mostly known for its anti-sigma factor activity for σF, it has been assigned the same role for σG 136 , 143, albeit this activity is primarily restricted to the mother cell 142. Furthermore, premature activation of σG in the prespore has toxic effects 121 , 135 and is prevented by the dependency on σE activity in the mother cell 144. In this way, elimination of the inhibitory mechanism by Gin 119 , 120 (which is blocking the activation of σG) is delayed until engulfment is complete and the requirement for σG-directed gene expression is created.
Several proteins and mechanisms have been described that have important functions in the activation of σG in the forespore, including the eight SpoIIIA proteins (SpoIIIAA-SpoIIIAH), SpoIIQ and SpoIIIJ. A membrane channel consisting of SpoIIQ-SpoIIIAH proteins was shown to facilitate the movement of molecules between the forespore and the mother cell compartment and plays a crucial role in the delivery of a mother cell-specific signal required for the activation of σG 121, 131, 134. The exact reason for SpoIIIJ requirement in sporulation is not yet fully understood 145. Recently, it was reported that SpoIIIJ is important for interaction with the SpoIIIAE protein, which is required for further development of the spore upon completion of engulfment 122, 146. Nevertheless, the organization of these complexes as well as their precise role in the activation of σG remain to be elucidated.
Once activated, a positive feedback loop in which σG recognizes its own promoter leads to an accumulation of this forespore-specific sigma factor and instigates transcription of genes within the σG regulon 117. Consecutively, levels of the inhibitory protein CsfB in the forespore decrease rapidly after activation of σG 120. It is so far unknown whether this is due to repression of csfB transcription, CsfB protein degradation or a combination of both.
The σG regulon comprises approximately 95 genes, a subset of which are co-controlled by the transcriptional regulator SpoVT whose production is also directed by σG 101. Functional classes of genes belonging to the σG regulon include sporulation, germination and protecting the spore from DNA damage 33. On the regulation level, σG activates the transcription of spoIVB that forms a signal for activation of σK in the mother cell, bofC, required for activation of pro-SigK and spoVT