ASSEMBLY OF THE FTSZ RING AT THE CENTRAL DIVISION SITE IN THE ABSENCEOF THE CHROMOSOME

The FtsZ ring assembles between segregated daughter chromosomes in pro karyotic cells and is essential for cell division. To understand bette r how the FtsZ ring is influenced by chromosome positioning and struct ure in Escherichia coil, we investigated its localization in parC and mukB mutants that are defective for chromosome segregation. Cells of b oth mutants at non-permissive temperatures were either filamentous wit h unsegregated nucleoids or short and anucleate. In parC filaments, Ft sZ rings tended to localize only to either side of the central unsegre gated nucleoid and rarely to the cell midpoint; however, medial rings reappeared soon after switching back to the permissive temperature. Fi lamentous mukB cells were usually longer and lacked many potential rin gs. At temperatures permissive for mukB viability, medial FtsZ rings a ssembled despite the presence of apparently unsegregated nucleoids. Ho wever, a significant proportion of these FtsZ rings were mislocalized or structurally abnormal. The most surprising result of this study was revealed upon further examination of FtsZ ring positioning in anuclea te cells generated by the parC and mukB mutants: many of these cells, despite having no chromosome, possessed FtsZ rings at their midpoints. This discovery strongly suggests that the chromosome itself is not re quired for the proper positioning and development of the medial divisi on site.