Stimulation of the Streptococcus pneumoniae RecA protein-promoted three-strand exchange reaction by the competence-specific SsbB protein

The effect of the transformational competence-specific Streptococcus pneumoniae single-stranded DNA binding protein, SpSsbB, on the ATP-dependent three-strand exchange activity of the SpRecA protein was investigated. Although SpRecA exhibited only a trace level of strand exchange activity in the absence of SpSsbB, an extensive strand exchange reaction was observed when SpSsbB was added to the reaction solution after SpRecA. A more limited strand exchange reaction was observed, however, when SpSsbB was added to the reaction solution before SpRecA. This dependence on the order of addition, together with additional DNA-dependent ATP hydrolysis experiments, indicated that the mechanism of stimulation may involve the postsynaptic binding of SpSsbB to the displaced linear single-stranded DNA reaction product. When dATP was provided in place of ATP as the nucleotide cofactor (to suppress a potentially inhibitory effect of SpSsbB on the interaction of SpRecA with the circular ssDNA reaction substrate), the stimulatory effect of SpSsbB on the strand exchange reaction was apparent regardless of the order in which it was added to the reaction solution. These findings suggest that SpSsbB may be able to facilitate SpRecA-promoted DNA recombination reactions during natural transformation in S. pneumoniae.

► The effect of SpSsbB on the SpRecA-promoted strand exchange reaction was investigated. ► SpRecA had only a minimal level of strand exchange activity in the absence of SpSsbB. ► An extensive strand exchange reaction was observed if SpSsbB was added after SpRecA. ► SpSsbB may serve as a stimulatory factor for SpRecA-mediated recombination reactions.