DNA Segregation in Bacteria

Abstract

Segregation of DNA in bacterial cells is an efficient process that assures that every daughter cell receives a copy of genomic and plasmid DNA. In this review, we focus primarily on observations in recent years, including the visualization of DNA and proteins at the subcellular level, that have begun to define the events that separate DNA molecules. Unlike the process of chromosome segregation in higher cells, segregation of the bacterial chromosome is a continuous process in which chromosomes are separated as they are replicated. Essential to separation is the initial movement of sister origins to opposite ends of the cell. Subsequent replication and controlled condensation of DNA are the driving forces that move sister chromosomes toward their respective origins, which establishes the polarity required for segregation. Final steps in the resolution and separation of sister chromosomes occur at the replication terminus, which is localized at the cell center. In contrast to the chromosome, segregation of low-copy plasmids, such as Escherichia coli F, P1, and R1, is by mechanisms that resemble those used in eukaryotic cells. Each plasmid has a centromere-like site to which plasmid-specified partition proteins bind to promote segregation. Replication of plasmid DNA, which occurs at the cell center, is followed by rapid partition protein-mediated separation of sister plasmids, which become localized at distinct sites on either side of the division plane. The fundamental similarity between chromosome and plasmid segregation-placement of DNA to specific cell sites-implies an underlying cellular architecture to which both DNA and proteins refer.

Molecular Basis of the Supercoil Deficit Induced by the Mini-F Plasmid Partition Complex

“…Both enzyme occlusion and supercoil reduction were attributed to an ability of the SopB-sopC complex to nucleate formation of a larger complex which through SopB-SopB interaction and nonspecific DNA-SopB binding extends over neighboring DNA. While spreading of the partition complex has been deemed a sufficient basis for enzyme occlusion, reduction of negative supercoiling has been assumed to require in addition the wrapping of DNA on SopB to form local positive supercoils (45)(46)(47). Our present results do not support this explanation for the supercoil deficit in mini-F plasmids.…”

Chromosome segregation control by Escherichia coli ObgE GTPase

“…After replication at mid-cell, the origin region (oriC) is rapidly segregated outward. The speed at which this occurs (reviewed in Gordon and Wright, 2000) rules out passive models for bacterial chromosome segregation, which proposed that outward cellular growth could drive the movement of a fixed chromosome. As the loci of the chromosome are replicated, they are moved outward to the poles in a sequential fashion (Lau et al, 2003;Viollier et al, 2004;Bates and Kleckner, 2005;Nielsen et al, 2006).…”

Towards understanding the molecular basis of bacterial DNA segregation

“…However, recent experiments revealing that the origins of bacterial chromosomes rapidly separate, in a manner independent of cell elongation, have rendered this model invalid. The finding that DNA replication in Bacillus subtilis and Escherichia coli probably occurs at a stationary, centrally located replication factory has led to the proposal that bi-directional extrusion of newly replicated DNA from the replication factory followed by DNA condensation might constrain the motion of sister nucleoids to opposite sides of the division plane (Gordon & Wright 2000;Koppes et al 1999;Lemon & Grossman 1998, 2000Onogi et al 2002;Sawitzke & Austin 2001). Transertion (coupled transcription, translation and insertion) of membrane proteins has been speculated to have a role in chromosome segregation (Norris 1995;Woldringh 2002) and more recently, Dworkin & Losick (2002) have proposed RNA polymerase as a novel candidate for driving the poleward motion of segregating bacterial chromosomes.…”