SigK Commitment to sporulation and engulfment
SigK during commitment to sporulation
Under the control of both σE and SpoIIID, pro-σK is synthesized in the mother cell during the engulfment process 123. The sigK gene is composed of two sections made up by two truncated genes called spoIIIC and spoIVCB that are separated from each other by a genetic element called skin (sigmaK intervening) 124. During sporulation, and mediated by mother cell-specific recombinase SpoIVCA, site-specific recombination of the DNA at this locus results in the excision of the skin element and rearrangement of the two truncated genes into one coding sigK gene 125, 126. Transcription of this sigK gene gives rise to an inactive precursor (pro-σK) that localizes to the membrane 127 and requires posttranslational processing before it can execute its activity in transcription of genes that are part of the σK regulon 128. Crucial checkpoints in the completion of the morphological stage of engulfment as well as intercellular communication ensure avoidance of premature processing, as this event depends on σG activity in the forespore. In the absence of these genetic and posttranslational control systems, inappropriate synthesis, activity and accumulation of σK result in toxicity and cell lysis 129.