Table 1.

Comparisons of the proposed segregation factors

Segregation factors	Remarks	Our interpretation
Chromosome tethering to the membrane and growth55	The replicated ori moves much faster than the elongation rate of the cell24	This may play a part in maintaining the DNA in the concentric shell
MreB56 (an actin homologue)	Escherichia coli and filamentous cyanobacteria are viable without MreB57,58 For Caulobcter crescentus, the role of MreB is conditional27 A recent study shows the functional interaction between MreB and Topo IV40	This could contribute to efficient segregation by maintaining a high aspect ratio of the cell
ParM6 (an actin homologue)	This has been described as a factor involved in plasmid segregation	ParM is not conserved in all bacteria and probably only functions for plasmids
ParA27	This has been described as a factor involved in plasmid segregation For chromosomes, it has a proposed role in ori movement in the initial stage of the cell cycle in C. crescentus and Vibrio cholerae 25,27 There is no evidence for a role in bulk chromosome segregation	This may help to extrude the newly synthesized DNA to the periphery of the nucleoid It is particularly helpful for the initial separation of the duplicated ori, when there is not enough DNA-scarce space in the periphery of the nucleoid in some organisms (for example, C. crescentus)
migS and the bacterial ‘centromere’ (REF. 59)	The mutant has a minor phenotype59	It is unlikely that the putative bacterial ‘centromere’ plays a notable role in bulk chromosome segregation
MukB60 and SMC proteins	Cells are usually conditionally viable without them4,23	These proteins increase the correlation length of the chromosome to promote efficient segregation They have interchangeable roles with other factors that increase the repulsive interactions between DNA (for example, an increased level of negative supercoiling)
Extrusion–capture30	Replication forks are independent in slow-growing E. coli cells31,32	This may help to extrude the newly synthesized DNA to the periphery of the nucleoid
RNA polymerase34	Its role can be tested by the run-off DNA synthesis experiment (that is, by blocking protein synthesis)	This may help to extrude the newly synthesized DNA to the periphery of the nucleoid
Coupling of transcription and translation, and membrane transertion35	Its role can be tested by the run-off DNA synthesis experiment (that is, by blocking protein synthesis)	This may help to extrude the newly synthesized DNA to the periphery of the nucleoid
Mechanical pushing (can be induced by cohesion)47,48	Although they differ in their biological details, this model has the same view as the entropy model: that the intrinsic physical and mechanical properties of the chromosome are important for its biological function	This can be understood in terms of excluded-volume interaction and conformational entropy
Excluded-volume interactions, chain-connectivity and conformational entropy15	The phase diagram presented in FIG. 1 explains the demixing of individual bulk chromosomes but does not consider the segregation of multiple chromosomes Current work should be extended to incorporate the effect of chromosome topology (for example, the branched structure of supercoiled plectonemes)	NA

NA, not applicable; ori, origin; Par, plasmid partition protein; SMC, structural maintenance of chromosomes.