Experiments on movement of DNA regions in Escherichia coli evaluated by computer simulation

During the cell cycle of Escherichia coli DNA is replicated and segregated over two prospective daughter cells. Nucleoids as a whole separate graduall y in line with cell elongation, but sub-nucleoid DNA regions may behave dif ferently, separating non-gradually. We tested the ability of three models t o predict the outcome of a fluorescent in situ hybridisation (FISH) experim ent. We did this by comparing computer-simulated data with experimental dat a. The first model predicts gradual separation in line with cell elongation , The second model predicts that origins stick together for some time after duplication before one copy jumps to the other side of the cell (non-gradu al separation). The simulated data of these models are very similar, indica ting that FISH is not a suitable method to distinguish between these two mo dels. The third model predicts that origins may be anywhere within the nucl eoid(s). We found that simulated data using the third model resemble the ex perimental data most. However, DNA regions are not randomly localised in th e cell. although their localisation is fuzzy. We propose that movement of D NA regions is the result of a combination of factors. Nucleoid segregation (or the forces behind it) dictates the overall direction of movement. Other factors. of which we show that diffusion could be an important one, move D NA in other directions giving rise to non-gradual movement in individual ce lls and contributing to variation in intracellular position per cell length in a population of cells. (C) 2001 Societe francaise de biochimie et biolo gie moleculaire / Editions scientifiques et medicales Elsevier SAS.