Localization of FtsL to the Escherichia coli septal ring

In Escherichia coli, nine gene products are known to be essential for assem bly of the division septum. One of these, FtsL, is a bitopic membrane prote in whose precise function is not understood. Here we use fluorescence micro scopy to study the subcellular localization of FtsL, both in a wild-type st rain and in a merodiploid strain that expresses a GFP-FtsL fusion protein. We show that FtsL localizes to the cell septum where it forms a ring analog ous to the cytoplasmic FtsZ ring. FtsL localization is dependent upon the f unction of FtsZ, FtsA and FtsQ, but not Ftsl. In a reverse approach, we use fusions of green fluorescent protein (GFP) to FtsZ, FtsA and ZipA to show that these proteins localize to the division site in an FtsL-independent fa shion. We propose that FtsL is a relatively late recruit to the ring struct ure that mediates septation.