Localization of F plasmid SopB protein to positions near the poles of Escherichia coli cells.

The subcellular localization of the SopB protein, which is encoded by the Escherichia coli F plasmid and is involved in the partition of the single-copy plasmid, was directly visualized through the expression of the protein fused to the jellyfish green fluorescent protein (GFP). The fusion protein, but not GFP itself, was found to localize to positions close but not at the poles of exponentially growing cells. Neither the presence of other F-encoded proteins nor the binding of SopB to its recognition sites within the sopC locus of F is required for this localization. Examination of derivatives of the fusion protein lacking various regions of SopB suggests that the signal for the cellular localization of SopB resides in a region close to its N terminus. It is plausible that the near polar localization of SopB may serve the function of keeping a segregated pair of F plasmids apart while the cell septum is being formed. The plausible relation between the specific location of SopB and its suppression of sopC-linked genes when overexpressed is also discussed.