SigF Commitment to sporulation and engulfment
SigF during commitment to sporulation
Activation of forespore-specific sigma factor σF is the first step towards further development of the forespore. During the formation of the asymmetric septum σF is held inactive through direct binding by SpoIIAB and phosphorylation of SpoIIAA. Next to its role in the formation of the polar septum (also see the ONSET state), SpoIIE also plays a crucial role in the activation of σF after asymmetric cell division is complete. It acts by dephosphorylating and hence activating SpoIIAA in the forespore compartment, which releases σF from the SpoIIAB-σF complex 98. SpoIIE is known to alternate its localization between the septal membrane and the membrane surrounding the forespore, but relocalizes to the asymmetric division site upon engulfment 99. This relocalization is an important determinant in preventing premature activation of σF and furthermore ensures confined σF activation in the forespore only. Complemented by genetic asymmetry and the chromosomal position of the spoIIA operon 86, spatial activation of σF signifies the start of compartmentalized gene expression. Posttranslational regulation of σF involves a complex biochemical mechanism in which phosphorylation, dephosphorylation and ATP/ADP levels seem to play critical roles. For more extensive reviews on biochemical evidence, possible explanations and proposed mechanisms for the regulation of σF we refer to references 33 and 66.
Once activated, σF is responsible for switching on a forespore-specific gene set consisting of approximately 70 genes > 100, 101. Some of these encode proteins that are important for further progression of the sporulation process, such as spoIIR (required for the activation of mother cell-specific σE), sigG (encoding late stage forespore-specific sigma factor σG), csfB (required for proper timing of σG activation), spoIVB (required for activation of late stage mother cell-specific σK) and rsfA (involved in σF-dependent gene transcription).