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 oriC s 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.
The enzyme S-adenosylmethionine (SAM) synthetase, the Escherichia coli metK gene product, produces SAM, the cell's major methyl donor. We show here that SAM synthetase activity is induced by leucine and repressed by Lrp, the leucine-responsive regulatory protein. When SAM synthetase activity falls below a certain critical threshold, the cells produce long filaments with regularly distributed nucleoids. Expression of a plasmid-carried metK gene prevents filamentation and restores normal growth to the metK mutant. This indicates that lack of SAM results in a division defect.
To study the role of cell division in the process of nucleoid segregation, we measured the DNA content of individual nucleoids in isogenic Escherichia coli cell division mutants by image cytometry. In pbpB (Ts) and ftsZ strains growing as filaments at 42°C, nucleoids contained, on average, more than two chromosome equivalents compared with 1.6 in wild‐type cells. Because similar results were obtained with a pbpB recA strain, the increased DNA content cannot be ascribed to the occurrence of chromosome dimers. From the determination of the amount of DNA per cell and per individual nucleoid after rifampicin inhibition, we estimated the C and D periods (duration of a round of replication and time between termination and cell division respectively), as well as the D′ period (time between termination and nucleoid separation). Compared with the parent strain and in contrast to ftsQ , ftsA and ftsZ mutants, pbpB (Ts) cells growing at the permissive temperature (28°C) showed a long D′ period (42 min versus 18 min in the parent) indicative of an extended segregation time. The results indicate that a defective cell division protein such as PbpB not only affects the division process but also plays a role in the last stage of DNA segregation. We propose that PbpB is involved in the assembly of the divisome and that this structure enhances nucleoid segregation.
Proper positioning of division sites in Escherichia coli requires balanced expression of minC, minD, and minE gene products. Previous genetic analysis has shown that either MinD or an apparently unrelated protein, DicB, cooperates with MinC to inhibit division. We have isolated and sequenced minC mutations that suppress division inhibition caused by overproduction of either DicB or MinD proteins. Most missense mutations were located in the amino acid 160 to 200 region of MinC (231 amino acids). Some mutations exhibited preferential resistance to one or the other coinhibitor, suggesting that two distinct proteins, possibly MinD and DicB themselves, interact in slightly different manners with the same region of MinC to promote division inhibition.
