SigK Completion of engulfment and cortex systhesis
SigK during engulfment
Previously synthesized pro-σK resides in the membrane where it awaits proteolytic modification that will result in activation of σK 127. The signal for such processing derives from the forespore compartment in the form of SpoIVB and is dependent on the activity of σG 147. Upon reception of this signal, inhibition of the mother cell-derived putative metalloprotease SpoIVFB by SpoIVFA is lifted so it can execute its function in converting pro-σK to σK 33, 148, 149, a process that depends on ATP levels 150.
Activity of σK in the mother cell marks the final stages of regulation in gene transcription directed towards the formation and maturation of the spore. Together with additional regulator protein GerE, σK is responsible for the transcription of a large set of structural spore coat and cortex protein-encoding genes 100. Furthermore, the expression of early sporulation genes is downregulated by the emergence of negative feedback loops within the mother cell. Firstly, through a yet unknown mechanism, the prolonged activity of house-keeping sigma factor σA is repressed by σK, leading to reduced levels of early mother cell-specific sigma factor σE and transcriptional regulator SpoIIID 137, 151. Secondly, σK levels are also kept low by the repressing action of GerE 152, although this feedback regulation was found to be non-essential for proper spore maturation and described to merely have a fine-tuning effect 153. Elimination of the first negative feedback loop, however, was found to cause severe spore defects and seems to comprise an essential event in spore formation 155. Through these regulatory feedback loops as well as through proposed differences in σK sensitivity of late stage sporulation promoters 129, activity of genes part of the σK regulon is allocated to specific time slots.