Nucleoid partitioning and the division plane in Escherichia coli

Escherichia coli nucleoids were visualized after the DNA of OsO4-fixed but hydrated cells was stained with the fluorochrome DAPI (4',6-diamidino-2-phenylindole dihydrochloride hydrate). In slowly growing cells, the nucleoids are rod shaped and seem to move along the major cell axis, whereas in rapidly growing, wider cells they consist of two- to four-lobed structures that often appear to advance along axes lying perpendicular or oblique to the major axis of the cell. To test the idea that the increase in cell diameter following nutritional shift-up is caused by the increased amount of DNA in the nucleoid, the cells were subjected to DNA synthesis inhibition. In the absence of DNA replication, the nucleoids continued to move in the growing filaments and were pulled apart into small domains along the length of the cell. When these cells were then transferred to a richer medium, their diameters increased, especially in the region enclosing the nucleoid. It thus appears that the nucleoid motive force does not depend on DNA synthesis and that cell diameter is determined not by the amount of DNA per chromosome but rather by the synthetic activity surrounding the nucleoid. Under the non-steady-state but balanced growth conditions induced by thymine limitation, nucleoids become separated into small lobules, often lying in asymmetric configurations along the cell periphery, and oblique and asymmetric division planes occur in more than half of the constricting cells. We suggest that such irregular DNA movement affects both the angle of the division plane and its position.