The extrusion-capture model for chromosome partitioning in bacteria

Successful cellular reproduction requires accurate duplication and partitioning (segregation) of the genome. Failure to correctly partition the sister genomes results in aneuploidy. The consequences of these errors range from loss of normal cellular function (e.g., the loss of normal growth controls in tumor cells) to cell death. In prokaryotes with a single chromosome, partitioning failures are fatal for at least one of the two daughter cells; a so-called anucleate cell forms when one daughter receives no chromosome and the other daughter receives two chromosomes.

Several findings in recent years have fundamentally altered our view of chromosome partitioning in prokaryotes (for review, see Gerdes et al. 2000; Gordon and Wright 2000; Hiraga 2000; Møller-Jensen et al. 2000;Donachie 2001; Sawitzke and Austin 2001). The flurry of new observations was ignited by adaptation of cell biological techniques used in eukaryotes (immunofluorescence, GFP, and fluorescent in situ hybridization) for use in prokaryotes (Harry et al. 1995; Pogliano et al. 1995; Webb et al. 1995; Niki and Hiraga 1998). This review focuses on chromosome partitioning in bacteria that have a single circular chromosome, specifically, the gram-positive organism, Bacillus subtilis, and the gram-negative organisms, Escherichia coli and Caulobacter crescentus. In these bacteria, DNA replication initiates once per cell division cycle from a specific chromosomal locus, oriC, and proceeds bidirectionally to terminate in a defined region opposite the origin, terC (Fig.1). The basic components of the DNA replication machinery are highly conserved in bacteria (Kornberg and Baker 1992); in fact, these basic components are functionally conserved from bacteria to mammals (Baker and Bell 1998).