Figure 3.

Schematic model to explain the role of the StbAB system.²¹ Open and shaded regions indicate nucleoid and cytosol spaces, respectively. Blue arrows indicate segregation of the host chromosome, and red arrows represent conjugative transfer to recipient cells (light red nucleoid). Plasmid R388 molecules are evenly distributed both in nucleoid and cytosol areas. At cell division, each daughter cell contains plasmid copies. Localization of plasmid copies at the cell pole is correlated with R388 capacity to undergo conjugative transfer. In contrast, DNA molecules of plasmid R388ΔstbA are exclusively localized in cytosol spaces toward the cell poles and the cell center. This is correlated with plasmid instability, since cells containing all copies in one side of the cell give rise to plasmid-free cells (dark cytoplasm), and with high conjugation frequency, since there are more plasmid copies at the poles. DNA molecules of plasmid R388ΔstbB are distributed in nucleoid but not in cytosol spaces, which correlates with a defect in conjugative transfer.