Cell Division inBacillus subtilis: FtsZ and FtsA Association Is Z-Ring Independent, and FtsA Is Required for Efficient Midcell Z-Ring Assembly

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Abstract
The earliest stage in cell division in bacteria is the assembly of a Z ring at the division site at midcell. Other division proteins are also recruited to this site to orchestrate the septation process. FtsA is a cytosolic division protein that interacts directly with FtsZ. Its function remains unknown. It is generally believed that FtsA localization to the division site occurs immediately after Z-ring formation or concomitantly with it and that FtsA is responsible for recruiting the later-assembling membrane-bound division proteins to the division site. Here, we report the development of an in vivo chemical cross-linking assay to examine the association between FtsZ and FtsA in Bacillus subtilis cells. We subsequently use this assay in a synchronous cell cycle to show that these two proteins can interact prior to Z-ring formation. We further show that in a B. subtilis strain containing an ftsA deletion, FtsZ localized at regular intervals along the filament but the majority of Z rings were abnormal. FtsA in this organism is therefore critical for the efficient formation of functional Z rings. This is the first report of abnormal Z-ring formation resulting from the loss of a single septation protein. These results suggest that in this organism, and perhaps others, FtsA ensures recruitment of the membrane-bound division proteins by ensuring correct formation of the Z ring.