Bacterial DNA segregation by dynamic SopA polymers

Most bacterial chromosomes contain homologs of plasmid partitioning (par) loci. These loci encode ATPases called ParA that are thought to contribute to the mechanical force required for chromosome and plasmid segregation. In Vibrio cholerae, the chromosome II (chrII) par locus is essential for chrII segregation. Here, we found that purified ParA2 had ATPase activities comparable to other ParA homologs, but, unlike many other ParA homologs, did not form high molecular weight complexes in the presence of ATP alone. Instead, formation of high molecular weight ParA2 polymers required DNA. Electron microscopy and three-dimensional reconstruction revealed that ParA2 formed bipolar helical filaments on double-stranded DNA in a sequence-independent manner. These filaments had a distinct change in pitch when ParA2 was polymerized in the presence of ATP versus in the absence of a nucleotide cofactor. Fitting a crystal structure of a ParA protein into our filament reconstruction showed how a dimer of ParA2 binds the DNA. The filaments formed with ATP are left-handed, but surprisingly these filaments exert no topological changes on the right-handed B-DNA to which they are bound. The stoichiometry of binding is one dimer for every eight base pairs, and this determines the geometry of the ParA2 filaments with 4.4 dimers per 120 Å pitch left-handed turn. Our findings will be critical for understanding how ParA proteins function in plasmid and chromosome segregation.nucleoprotein filament | DNA segregation

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 94%

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ParA2, aVibrio choleraechromosome partitioning protein, forms left-handed helical filaments on DNA

Hui

Galkin²,

Xiong³

et al. 2010

“…ParA proteins interact with the ParB-parS complex, and their ATPase activity is essential for plasmid partitioning (5,6). Several ParA homologues have been shown to form ATPdependent polymers in vitro and to oscillate in vivo (7)(8)(9)(10). Still, the molecular mechanisms by which the interactions of ParA with the ParB-parS complex mediate plasmid localization and partitioning are not well understood, although several models have been proposed to account for Par-mediated plasmid segregation (9-13).…”

mentioning

confidence: 99%

par genes and the pathology of chromosome loss in Vibrio cholerae

Yamaichi¹,

Fogel

Waldor

2007

107

The causes and consequences of chromosome loss in bacteria with multiple chromosomes are unknown. Vibrio cholerae, the causative agent of the severe diarrheal disease cholera, has two circular chromosomes. Like many other bacterial chromosomes, both V. cholerae chromosomes contain homologues of plasmid partitioning (par) genes. In plasmids, par genes act to segregate plasmid molecules to daughter cells and thereby ensure plasmid maintenance; however, the contribution of par genes to chromosome segregation is not clear. Here, we show that the chromosome II parAB2 genes are essential for the segregation of chromosome II but not chromosome I. In a parAB2 deletion mutant, chromosome II is mislocalized and frequently fails to segregate, yielding cells with only chromosome I. These cells divide once; their progeny are not viable. Instead, chromosome II-deficient cells undergo dramatic cell enlargement, nucleoid condensation and degradation, and loss of membrane integrity. The highly consistent nature of these cytologic changes suggests that prokaryotes, like eukaryotes, may possess characteristic death pathways.chromosome segregation ͉ parA ͉ parB ͉ apoptosis

“…The first task of a partition system is to identify its DNA cargo. SopB accomplishes this task by loading onto sopC and forming a partition complex, which has been visualized in vivo by fluorescence microscopy as punctuate foci (7,8). The partition complex is believed to contain a large number of SopB dimers, some bound specifically to sopC and additional dimers bound near sopC (9-11).…”

mentioning

confidence: 99%

Cell-free study of F plasmid partition provides evidence for cargo transport by a diffusion-ratchet mechanism

Vecchiarelli

Hwang

Mizuuchi

2013

138

185

Increasingly diverse types of cargo are being found to be segregated and positioned by ParA-type ATPases. Several minimalistic systems described in bacteria are self-organizing and are known to affect the transport of plasmids, protein machineries, and chromosomal loci. One well-studied model is the F plasmid partition system, SopABC. In vivo, SopA ATPase forms dynamic patterns on the nucleoid in the presence of the ATPase stimulator, SopB, which binds to the sopC site on the plasmid, demarcating it as the cargo. To understand the relationship between nucleoid patterning and plasmid transport, we established a cell-free system to study plasmid partition reactions in a DNA-carpeted flowcell. We observed depletion zones of the partition ATPase on the DNA carpet surrounding partition complexes. The findings favor a diffusion-ratchet model for plasmid motion whereby partition complexes create an ATPase concentration gradient and then climb up this gradient toward higher concentrations of the ATPase. Here, we report on the dynamic properties of the Sop system on a DNA-carpet substrate, which further support the proposed diffusion-ratchet mechanism.bacterial chromosome segregation | ParA ATPase | plasmid segregation | spatial pattern organization | chromosome dynamics P roper DNA segregation ensures the faithful inheritance of genomic information for all life forms. In bacteria, this fundamental process is poorly understood. Low-copy bacterial genomes, including plasmids and chromosomes, encode active partition (Par) systems to ensure stability. Par systems are minimalistic in that only three dedicated components are required: a partition site on the DNA, a partition site-binding protein, and a nucleoside triphosphatase (NTPase). Par systems have been classified according to the type of NTPase involved: Walker-type (generically called "ParA"), actin-like, or tubulin-like (reviewed in ref. 1). Reconstitution of purified Par components of R1 plasmid in a cell-free system unveiled the mechanism involving an actin-like ATPase, ParM, in which elongating filaments of the ATPase push plasmids to opposite cell poles (2). Tubulin-like GTPases also appear to function as a filament (3). However, all chromosome-based and most plasmid-based systems use ParAs, and mounting evidence shows that ParA-like ATPases also are responsible for transporting large protein machineries (reviewed in ref. 4). However, the underlying mechanism for reactions of this category remains unresolved.The Sop system (stability of plasmid) of F plasmid is one of the first Par systems to be identified (5, 6) and is considered a paradigm for the study of ParA-mediated DNA segregation. The three plasmid-encoded system components are SopA (the ParAtype ATPase), SopB (or ParB in other systems; i.e., the partition site-binding protein), and sopC (or parS in other systems; i.e., the cis-acting partition site on the plasmid). The first task of a partition system is to identify its DNA cargo. SopB accomplishes this task by loading onto sopC and forming a partition c...