Nucleoid-binding protein RicO anchors replication origins to the membrane to ensure correct chromosome segregation in Staphylococcus aureus

The mechanisms underlying chromosome segregation in bacteria, particularly in non-canonical models, remain incompletely understood. The coccoid bacterium Staphylococcus aureus encodes a limited set of conserved proteins involved in chromosome segregation, including the Structural Maintenance of Chromosomes (SMC) complex and an incomplete partition system. Here, we identified a protein of previously unknown function that interacts with the nucleoid and ensures accurate chromosome segregation in S. aureus. RicO (Regulator and Insulator of Chromosomal Origins) contains a conserved N-terminal DNA-binding domain that recognizes a specific sequence motif in the origin-proximal region of the chromosome, as well as a C-terminal amphipathic helix that interacts with the cell membrane, thereby bridging DNA and the membrane. RicO localizes in membrane-proximal foci at the tip of each hemisphere (or cell poles). Cells lacking RicO fail to correctly position replication origins at the cell poles and have weakened genomic interactions in the origin-proximal region, resulting in chromosome segregation defects. The combined absence of RicO and a functional SMC complex leads to nearly half of the population appearing as anucleate cells, indicating that these two systems are key contributors to chromosome segregation in S. aureus. Collectively, our data support a model in which RicO anchors chromosomal origins at the cell periphery to ensure genome maintenance in this spherical bacterial pathogen.