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The conjugative plasmid R388 and a number of other plasmids carry an operon,  stbABC , adjacent to the origin of conjugative transfer. We investigated the role of the  stbA ,  stbB , and  stbC  genes. Deletion of  stbA  affected both conjugation and stability. It led to a 50-fold increase in R388 transfer frequency, as well as to high plasmid loss. In contrast, deletion of  stbB  abolished conjugation but provoked no change in plasmid stability. Deletion of  stbC  showed no effect, neither in conjugation nor in stability. Deletion of the entire  stb  operon had no effect on conjugation, which remained as in the wild-type plasmid, but led to a plasmid loss phenotype similar to that of the R388Δ stbA  mutant. We concluded that StbA is required for plasmid stability and that StbA and StbB control conjugation. We next observed the intracellular positioning of R388 DNA molecules and showed that they localize as discrete foci evenly distributed in live  Escherichia coli  cells. Plasmid instability of the R388ΔΔ stbA  mutant correlated with aberrant localization of the plasmid DNA molecules as clusters, either at one cell pole, at both poles, or at the cell center. In contrast, plasmid molecules in the R388ΔΔ stbB  mutant were mostly excluded from the cell poles. Thus, results indicate that defects in both plasmid maintenance and transfer are a consequence of variations in the intracellular positioning of plasmid DNA. We propose that StbA and StbB constitute an atypical plasmid stabilization system that reconciles two modes of plasmid R388 physiology: a maintenance mode (replication and segregation) and a propagation mode (conjugation). The consequences of this novel concept in plasmid physiology will be discussed.

The stb Operon Balances the Requirements for Vegetative Stability and Conjugative Transfer of Plasmid R388