Fig. 1.

Engulfment pathway of B. subtilis and cell-specific gene expression.

A. Asymmetric division allows activation of σ^F in the smaller forespore, followed by activation of s^E in the larger mother cell.

B. During engulfment, the mother cell membrane migrates around the forespore, meets (C) and fuses (D) to release the forespore into the mother cell cytoplasm. The forespore protein SpoIIQ is required to synthesize the second forespore transcription factor (σ^G) and for σ^G activation, which also requires the completion of engulfment and the mother cell proteins encoded by the spoIIIA operon (arrow in C). The σ^G factor then produces the secreted signal transduction protein SpoIVB (arrow in D), allowing σ^K activation.

E. Summary of checkpoints controlling late gene expression: engulfment directly controls σ^G activity, which produces SpoIVB, allowing σ^K activity.