ParABS

ParABS refers to a widely conserved bacterial DNA partitioning system that ensures faithful segregation of plasmids and, in some contexts, chromosomal regions during cell division. The system consists of three main elements: ParA, a deviant Walker-type ATPase; ParB, a DNA-binding protein; and parS, a centromere-like DNA sequence that serves as the loading site for ParB.

ParA functions as an ATP-dependent motor that binds nonspecifically to the nucleoid, forming a dynamic gradient. ParB binds specifically to parS on the plasmid or chromosomal region and, when in complex with parS, interacts with ParA. The interaction stimulates ATP hydrolysis by ParA, causing ParA to release from DNA and to rebind elsewhere, generating a moving complex of ParB-parS that biases the plasmid's position toward opposite cell poles or away from midcell.

This cycle can drive the segregation of replicated DNA copies before cytokinesis, providing near-equal inheritance of plasmids. ParABS systems are found on many low-copy-number plasmids and, in some bacteria, contribute to chromosome organization and segregation. Their components are conserved, though the exact mechanism and the spatial mode (whether pulling, pushing, or a diffusion-ratchet-type mechanism) can vary among species and plasmid types. ParA/ParB systems are studied as paradigms of intracellular movement driven by nucleotide-dependent protein dynamics and DNA-protein interactions, and they have practical implications for plasmid stability in biotechnology and for understanding bacterial cell biology.