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SigK Completion of engulfment and cortex systhesis (Stages III - IV)

SigK during engulfment

Previously synthesized pro-σ^K resides in the membrane where it awaits proteolytic modification that will result in activation of σ^{K  127}. The signal for such processing derives from the forespore compartment in the form of SpoIVB and is dependent on the activity of σ^{G 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-σ^K to σ^{K 33, 148, 149}, a process that depends on ATP levels  ¹⁵⁰.  

Activity of σ^K 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, σ^K 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 σ^A is repressed by σ^K, leading to reduced levels of early mother cell-specific sigma factor σ^E and transcriptional regulator SpoIIID ^{137, 151}. Secondly, σ^K 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 σ^K sensitivity of late stage sporulation promoters ¹²⁹, activity of genes part of the σ^K regulon is allocated to specific time slots.