Differential Gene Expression Governed by Chromosomal Spatial Asymmetry

The activity of the transcription factor σF is confined to one (the forespore) of two cells created by asymmetric division during sporulation in B. subtilis. We show that σF activation is partly governed by the position of the gene for the unstable anti-σF factor SpoIIAB. Because cytokinesis precedes chromosome segregation, most of the chromosome is translocated into the forespore after division. We hypothesize that because spoIIAB enters the forespore late, SpoIIAB lost to proteolysis is temporarily not replenished. Thus, chromosome asymmetry would be translated into the asymmetric distribution of SpoIIAB. Supporting this idea, transposition of spoIIAB to sites present in the forespore at the time of division impaired sporulation when a second pathway that participates in σF activation was disabled.