Assembly of the SpoIIIE DNA translocase depends on chromosome trapping in Bacillus subtilis

Sporulation in Bacillus subtilis is an attractive system in which to study the translocation of a chromosome across a membrane. Sporulating cells contain two sister chromosomes that are condensed in an elongated axial filament with the origins of replication anchored at opposite poles of the sporangium. The subsequent formation of a septum near one pole divides the sporangium unequally into a forespore (the smaller compartment) and a mother cell. The septum forms around the filament, trapping the origin-proximal region of one chromosome in the forespore. As a consequence, the trapped chromosome transverses the septum with the remainder being left in the mother cell. Next, SpoIIIE assembles at the middle of the septum to create a translocase that pumps the origin-distal, two-thirds of the chromosome into the forespore. Here, we address the question of how the DNA translocase assembles and how it localizes to the septal midpoint. We present evidence that DNA transversing the septum is an anchor that nucleates the formation of the DNA translocase. We propose that DNA anchoring is responsible for the assembly of other SpoIIIE-like DNA translocases, such as those that remove trapped chromosomes from the division septum of cells undergoing binary fission.