Segrosome Complex Formation during DNA Trafficking in Bacterial Cell Division

Abstract

Bacterial extrachromosomal DNAs often contribute to virulence in pathogenic organisms or facilitate adaptation to particular environments. The transmission of genetic information from one generation to the next requires sufficient partitioning of DNA molecules to ensure that at least one copy reaches each side of the division plane and is inherited by the daughter cells. Segregation of the bacterial chromosome occurs during or after replication and probably involves a strategy in which several protein complexes participate to modify the folding pattern and distribution first of the origin domain and then of the rest of the chromosome. Low-copy number plasmids rely on specialized partitioning systems, which in some cases use a mechanism that show striking similarity to eukaryotic DNA segregation. Overall, there have been multiple systems implicated in the dynamic transport of DNA cargo to a new cellular position during the cell cycle but most seem to share a common initial DNA partitioning step, involving the formation of a nucleoprotein complex called the segrosome. The particular features and complex topologies of individual segrosomes depend on both the nature of the DNA binding protein involved and on the recognized centromeric DNA sequence, both of which vary across systems. The combination of in vivo and in vitro approaches, with structural biology has significantly furthered our understanding of the mechanisms underlying DNA trafficking in bacteria. Here, I discuss recent advances and the molecular details of the DNA segregation machinery, focusing on the formation of the segrosome complex.

Keywords: DNA segregation; ParB; ParR; TubR; nucleoprotein complex; partitioning complex; partitioning systems; segrosome.

Figure 1

Scheme showing the structures of Type Ia CBPs domains and their interaction with DNA during bridging. (A) Central domain showing primary specific DNA interaction of ParB, SopB, Spo0J, and KorB. These proteins bind as dimers, making contact with both sides of the DNA. However, ParB dimerization generate a different contact mechanism that involves the bridging of different DNA molecules. (B) Spo0J N-terminal domain structures, both unbound and DNA-bound. Binding to the centromere induces a domain rearrangement that favors DNA bridging. (C) The C-terminal domain folding differs considerably between ParB/sopB and KorB. ParB folding includes extended loops that make contacts with DNA, favoring bridging distant molecules. DNA is shown in light purple, the HTH motif in blue, the N-terminal domain in light blue, and the C-terminal domain in dark blue.

Figure 2

Structural comparison of CBPs involved in segrosome assembly by wrapping. (A) Type II partition systems. Structure of ParR dimer (left), showing topology and the RHH motif; DNA changes upon ParR binding (middle), with enlargement of the DNA major groove; and formation of the the segrosome complex by DNA wrapping of the ParR super-helical oligomer (right), leaving the ParR C-terminal tail in the helix inside. (B) Type III partition systems. Structure of TubR dimer (left), showing topology and the HTH motif; the TubR-DNA binding mechanism (middle), in which the HTH makes contacts with the DNA major groove and the wing forms contacts with the minor groove; and putative filamentous vs. helical segrosome complexes (right), according to two crystal packing arrangements. (C) Type Ib partition systems. Structures of ParG and ω dimers (left), showing the RHH motif and the flexible N-terminal domain, and protein binding to direct and inverted repeats in equivalent ways (right).