The bypass of ZipA by overexpression of FtsN requires a previously unknown conserved FtsN motif essential for FtsA-FtsN interaction supporting a model in which FtsA monomers recruit late cell division proteins to the Z ring.

Assembly of the divisome in Escherichia coli occurs in two temporally distinct steps. First, FtsZ filaments attached to the membrane through interaction with FtsA and ZipA coalesce into a Z ring at midcell. Then, additional proteins are recruited to the Z ring in a hierarchical manner to form a complete divisome, activated by the arrival of FtsN. Recently, we proposed that the interaction of FtsA with itself competes with its ability to recruit downstream division proteins (both require the IC domain of FtsA) and ZipA's essential function is to promote the formation of FtsA monomers. Here, we tested whether overexpression of a downstream division protein could make ZipA dispensable, presumably by shifting the FtsA equilibrium to monomers. Only overexpression of FtsN bypassed ZipA and a conserved motif in the cytoplasmic domain of FtsN was required for both the bypass and interaction with FtsA. Also, this cytoplasmic motif had to be linked to the periplasmic E domain of FtsN to bypass ZipA, indicating that linkage of FtsA to periplasmic components of the divisome through FtsN was essential under these conditions. These results are used to further elaborate our model for the role of FtsA in recruiting downstream division proteins.

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