Localization of the Escherichia coli cell division protein FtsI (PBP3) to the division site and cell pole

Ftsl, also known as penicillin-binding protein 3, is a transpeptidase required for the synthesis of peptidoglycan in the division septum of the bacterium, Escherichia coli, Ftsl has been estimated to be present at about 100 molecules per cell, well below the detection limit of immunoelectron microscopy. Here, we confirm the low abundance of Ftsl and use immunofluorescence microscopy, a highly sensitive technique, to show that Ftsl is localized to the division site during the later stages of cell growth. Ftsl was also sometimes observed at the cell pole; polar localization was not anticipated and its significance is not known. We conclude (i) that immunofluorescence microscopy can be used to localize proteins whose abundance is as low as approximately 100 molecules per cell; and (ii) that spatial and temporal regulation of Ftsl activity in septum formation is achieved, at least in part, by timed localization of the protein to the division site.