Nucleoid-binding protein RicO associates replication origins with the cell membrane in Staphylococcus aureus

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.