FIG. 1.
σK checkpoint. (A) Diagram of a sporangium in which the forespore (FS) has been pinched off as a protoplast within the mother cell (MC). (B) Expanded view of the two membranes separating the forespore and mother cell, depicting BofA, SpoIVFA, and SpoIVFB in the outermost membrane surrounding the forespore. The three proteins form a complex in which SpoIVFB is inactive prior to signaling from the forespore (4, 5, 16, 30, 31, 33). σG RNA polymerase transcribes spoIVB in the forespore (3), and SpoIVB is secreted into the space between the two membranes (38), where it activates the processing complex, leading to the production of σK in the mother cell.