The replicated ftsQAZ and minB chromosomal regions of Escherichia coli segregate on average in line with nucleoid movement

The average cellular positions of the ftsQAZ region (2 min) and the minB region (26.5 min) during the cell cycle was determined by fluorescent in situ hybridization using the position of oriC as a reference point. At the steady-state growth conditions used, newborn cells had replicated about 50% of the chromosome. By measuring the distances of the labelled oriCs with respect to mid-cell, we found two well-separated average oriC positions in cells of newborn length. These average oriC positions moved further apart along with cell elongation. The cellular position of the ftsQAZ gene region resembled the position of oriC, although its average position was closer to mid-cell. In contrast, a single minB focus was observed at cell birth. Separated minB foci appeared towards the end of DNA replication. The average positions of oriC, ftsQAZ and minB relative to each other fitted a model in which DNA replication takes place in the cell centre and subsequent gene regions pass sequentially through this centre. We have interpreted the polarized orientation of the studied gene regions as a consequence of the mode of DNA segregation.