Distinct architectural requirements for the parS centromeric sequence of the pSM19035 plasmid partition machinery

Three-component ParABS partition systems ensure stable inheritance of many bacterial chromosomes and low-copy-number plasmids. ParA localizes to the nucleoid through its ATP-dependent nonspecific DNA-binding activity, whereas centromere-like parS-DNA and ParB form partition complexes that activate ParA-ATPase to drive the system dynamics. The essential parS sequence arrangements vary among ParABS systems, reflecting the architectural diversity of their partition complexes. Here, we focus on the pSM19035 plasmid partition system that uses a ParB_pSM of the ribbon-helix-helix (RHH) family. We show that parS with four or more contiguous ParB_pSM-binding sequence repeats is required to assemble a stable ParA_pSM-ParB_pSM complex and efficiently activate the ParA_pSM-ATPase, stimulating complex disassembly. Disruption of the contiguity of the parS sequence array destabilizes the ParA_pSM-ParB_pSM complex and prevents efficient ATPase activation. Our findings reveal the unique architecture of the pSM19035 partition complex and how it interacts with nucleoid-bound ParA_pSM-ATP.