Bacterial Chromosome Segregation: Is There a Mitotic Apparatus?

A deceptively simple, yet long standing, puzzle in bacterial cell division is how newly replicated chromosomes are equally distributed between two progeny cells. For many years, beginning with the seminal Jacob, Brenner, and Cuzin paper (1963), microbial geneticists have reasoned that the cell membrane is likely to play a role in this process by providing a relatively stable support structure for the replicating chromosomes. Pieces of this puzzle have indeed implicated the membrane in the regulation of DNA replication initiation, but a membrane-associated mitotic apparatus that is able to position chromosomes three-dimensionally and implement directed chromosome movement has yet to be described. The critical questions that must be answered include: Are chromosomes spatially oriented with respect to the cell pole and the division site? Are they pulled towards the cell poles by an active mechanism, or divided as an inescapable consequence of cell growth and septation? These lead to the larger question: Will years of study of bacterial `mitosis' uncover a bacterial cytoskeleton and its accompanying molecular motors as is the case for eukaryotic mitosis? Visualization of subcellular protein location in bacterial cells is now providing the means to approach these questions directly. Two papers in this issue of Cell address the first question by providing direct evidence that the newly replicated chromosomes are aligned with their origins oriented toward the poles of the predivisional cell (Webb et al. 1997; Mohl and Gober 1997; Figure 1).