During the early stages of spore formation in Bacillus subtilis, asymmetric division precedes chromosome segregation, such that the forespore transiently contains only about one-third of the genetic material surrounding the origin of replication. Shortly after septum formation, the transcription factor sigma (F) initiates forespore-specific gene expression that is essential for the proteolytic activation of pro-sigma (E) in the neighbouring mother cell. Moving the sigma (F)-dependent spoIIR gene from its original origin-proximal position to an ectopic origin-distal site caused a delay in spoIIR transcription, as well as delays and reductions in the proteolytic activation of pro-sigma (E) and sigma (E)-directed gene expression. These defects correlated with the accumulation of disporic sporangia, thus reducing sporulation efficiency in a manner that depended upon the distance that spoIIR had been moved from the origin-proximal third of the chromosome. A significant proportion of disporic sporangia exhibited sigma (E) activity in their central compartment, indicating that delays and reductions in sigma (E) activation can lead to the formation of a second septum at the opposite pole. These observations support a model in which chromosomal spoIIR position temporally regulates sigma (E) activation, thereby allowing for the rapid establishment of mother cell-specific gene expression that is essential for efficient spore formation. The implications of these findings for cell type-specific gene expression during the early stages of spore formation in B. subtilis are discussed.
