The unstructured N-terminal tail of ParG modulates assembly of a quaternary nucleoprotein complex in transcription repression.

Abstract

ParG is the prototype of a group of small (<10 kDa) proteins involved in accurate plasmid segregation. The protein is a dimeric DNAbindingfactor,whichconsistsof symmetric paired C-terminal domains that interleave into a ribbon-helix-helix fold that is crucial for the interaction with DNA, and unstructured N-terminal domains of previously unknownfunction. HeretheParGproteinis shown to be a transcriptional repressor of the parFG genes. The protein assembles on its operator site initially as a tetramer (dimer of dimers) and, at elevated protein concentrations, as a pair of tetramers. Progressive deletion of the mobile N-terminal tails concomitantly decreased transcriptional repression by ParG and perturbed the DNA binding kinetics of the protein. The flexible tails are not necessary for ParG dimerization but instead modulate the organization of a higher order nucleoprotein complex that is crucial for proper transcriptional repression. This is achieved by transient associations between the flexible and folded domains in complex with the target DNA. Numerous ParG homologs encoded by plasmids of Gram-negative bacteria similarly are predicted to possess N-terminal disordered tails, suggesting that this is a commonfeatureofpartition operon autoregulation. The results provide new insights into the role of natively unfolded domains in protein function, the molecular mechanismsoftranscriptionregulation, andthecontrol of plasmid segregation.