The segregation of newly replicated chromosomes in bacterial cells is a highly coordinated spatiotemporal process. In the majority of bacterial species, a tripartite ParAB-parS system, composed of an ATPase (ParA), a DNA-binding protein (ParB), and its target(s) parS sequence(s), facilitates the initial steps of chromosome partitioning. ParB nucleates around parS(s) located in the vicinity of newly replicated oriCs to form large nucleoprotein complexes, which are subsequently relocated by ParA to distal cellular compartments. In this review, we describe the role of ParB in various processes within bacterial cells, pointing out interspecies differences. We outline recent progress in understanding the ParB nucleoprotein complex formation and its role in DNA segregation, including ori positioning and anchoring, DNA condensation, and loading of the structural maintenance of chromosome (SMC) proteins. The auxiliary roles of ParBs in the control of chromosome replication initiation and cell division, as well as the regulation of gene expression, are discussed. Moreover, we catalog ParB interacting proteins. Overall, this work highlights how different bacterial species adapt the DNA partitioning ParAB-parS system to meet their specific requirements.

Conjugation, besides transformation and transduction, is one of the main mechanisms of horizontal transmission of genetic information among bacteria. Conjugational transfer, due to its essential role in shaping bacterial genomes and spreading of antibiotics resistance genes, has been widely studied for more than 70 years. However, new and intriguing facts concerning the molecular basis of this process are still being revealed. Most recently, a novel family of conjugative relaxases (Mob proteins) was distinguished. The characteristic feature of these proteins is that they are not related to any of Mobs described so far. Instead of this, they share significant similarity to tyrosine recombinases. In this study MobK—a tyrosine recombinase-like Mob protein, encoded by pIGRK cryptic plasmid from the Klebsiella pneumoniae clinical strain, was characterized. This study revealed that MobK is a site-specific nuclease and its relaxase activity is dependent on both a conserved catalytic tyrosine residue (Y¹⁷⁹) that is characteristic of tyrosine recombinases and the presence of Mg²⁺ divalent cations. The pIGRK minimal origin of transfer sequence (oriT) was also characterized. This is one of the first reports presenting tyrosine recombinase-like conjugative relaxase protein. It also demonstrates that MobK is a convenient model for studying this new protein family.