Biophysical Studies of Cell Division Protein Localization Mechanisms in <i>Escherichia coli</i>

How nanometer-scale proteins position accurately within micron-scale bacteria has intrigued both biologists and physicists alike. A critical process requiring precise protein localization is cell division. In most bacteria, cell division starts with the self-assembly of the FtsZ proteins into filaments that form a ring-like structure encircling the cell at its middle, the Z-ring. The Z-ring is a scaffold for additional proteins that synthesize the lateral cell wall which separates the two daughter cells. If division planes are misplaced relative to bacterial chromosomes, also called nucleoids, daughter cells with incomplete genetic material can be produced. In Escherichia coli, research carried out over the past several decades has determined two independent molecular mechanisms that are involved in the midcell placement of the division plane, the Min system, and the SlmA proteins. By combining quantitative image analysis, fluorescence microscopy, and molecular biology techniques, this work provides evidence for two additional mechanisms that coordinate Z-ring positioning with chromosome segregation in E. coli. The first mechanism revealed itself in cells that had the Min system and the SlmA proteins removed. In these cells, the Z-ring invariably localized at the center of the nucleoid. Formation of Z-ring in this location depended on cell cycle dependent movement of the replication terminus region (Ter) to nucleoid middle, and on ZapA, ZapB, and MatP proteins. The second mechanism was revealed in cells where Z-rings were strongly misplaced relative to chromosomes. Interestingly, most of these cells were still viable. We determined that cells retained their viability because as much as 1/3 of the chromosome moved across the closing division plane in the late stages of cytokinesis. Chromosome repositioning appears to rely on septal cell wall synthesis rather than on DNA translocase activity. Altogether this work demonstrates that E. coli harbors several partially redundant molecular systems, in addition to those known previously, that collectively guarantee accurate and robust placement of both cell division proteins and chromosomes.