Role of the unstructured N-terminus of the centromere binding protein ParG in mediating segregation of the multidrug resistance plasmid TP228

TP228 is a large low copy number plasmid harbouring the parFGH partition cassette. The centromere-like site parH is located upstream of the parFG genes. ParF is a Walker-type ATPase of the ParA superfamily. ParG is a centromere binding protein and a transcriptional repressor of the parFG genes. ParF associates with ParG bound to parH forming the segrosome complex. It has been recently observed that ParF oscillates over the nucleoid in the presence of the entire parFGH system and oscillation is responsible for plasmid segregation. ParG is a dimeric protein: each monomer consists of a folded ribbon-helix-helix domain and an unstructured N-terminal tail. ParG enhances ParF ATPase activity and promotes ParF self-assembly through its flexible N-terminus.

In the present study, the role of the ParG N-terminus in plasmid partition was dissected. Residues crucial for plasmid partition were identified and found to form three clusters within the tail. One cluster is located at the extreme tip of the N-terminus that is the most flexible region. The second cluster is present in a linker-type region around amino acids 11-12-13 and the third is positioned in the arginine finger loop. When ParG mutant proteins were purified and characterised, they were all found to be efficient in DNA binding, transcriptional repression and in enhancing ParF polymerization. However, all the ParG mutants were impaired in stimulating ParF ATPase activity. Alteration of the residues in the tip and linker region resulted into a weaker interaction with ParF. The mutants were further investigated by using confocal and super resolution microscopy to visualize protein and plasmid positioning in the cell. Time-lapse experiments showed plasmids were static over time and that ParF oscillation over the nucleoid was abolished in the presence of mutant proteins. All the three clusters of the N-terminal tail are responsible for stimulating ParF ATPase activity and failure to do so may lead to lack of ParF oscillation. It is possible that the residues in the ParG N-terminus are strategically placed to carry out interaction and activation functions towards the common goal of coordinated interplay with ParF for efficient plasmid segregation. The data indicate that, a functional ParG N-terminal tail is a prerequisite for ParF oscillation and plasmid segregation. Based on these findings, a novel plasmid partition model is proposed which may apply to ParA-mediated partition in other plasmid systems.