The midcell replication factory in Bacillus subtilis is highly mobile: implications for coordinating chromosome replication with other cell cycle events

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Abstract
During vegetative growth, rod-shaped bacterial cells such as Escherichia coli and Bacillus subtilis divide precisely at midcell. It is the Z ring that defines the position of the division site. We previously demonstrated that the early stages of chromosome replication are linked to midcell Z ring assembly in B. subtilis and proposed a direct role for the centrally located replication factory in masking and subsequently unmasking the midcell site for Z ring assembly. We now show that the replication factory is significantly more scattered about the cell centre than the Z ring in both vegetative cells and outgrown spores of B. subtilis. This finding is inconsistent with the midcell replication factory acting as a direct physical block to Z ring assembly. Time-lapse experiments demonstrated that the lower precision of replication factory positioning results from its high mobility around the cell centre. Various aspects of this mobility are presented and the results are discussed in the light of current views on the determinants of positional information required for accurate chromosome segregation and cell division.