The parABS system is a broadly conserved molecular mechanism for

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...

partitioning and

chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

. Originally identified as a genetic element required for faithful partitioning of low-copy-number plasmids, it consists of three components: the ParA

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

, the ParB DNA-binding protein, and the cis-acting ''parS'' sequence. The ''parA'' and ''parB'' genes are typically found in the same

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

, with ''parS'' elements located within or adjacent to this operon. Collectively, these components function to ensure accurate partitioning of plasmids or whole chromosomes between bacterial daughter cells prior to cell division.

Mechanism

Based on

chromatin immunoprecipitation Chromatin immunoprecipitation (ChIP) is a type of immunoprecipitation experimental technique used to investigate the interaction between proteins and DNA in the cell. It aims to determine whether specific proteins are associated with specific genom ...

(ChIP) experiments, ParB has the ability to bind not only to high-affinity ''parS'' sites but also to adjacent nonspecific DNA, a behavior known as "spreading". The ParB-DNA complex is thought to be translocated by a Brownian ratchet mechanism involving the ParA ATPase: ParA binds DNA nonspecifically in its ATP-bound state but much more weakly in its ADP-bound state. The ParB-DNA complex binds to ATP-bound ParA, stimulating its ATPase activity and its dissociation from DNA. In this way, the ParB-DNA complex can be translocated by chasing a receding wave. This translocation mechanism has been observed by fluorescence microscopy both ''in vivo'' and more recently ''in vitro'' with purified components.

References

Plasmids {{microbiology-stub