Effect of the Streptococcus pneumoniae MmsA protein on the RecA protein-promoted three-strand exchange reaction

Streptococcus pneumoniae is a naturally transformable bacterium that is able to incorporate DNA from its environment into its own chromosome. This process, known as transformational recombination, is dependent in part on the mmsA gene, which encodes a protein having a sequence that is 40% identical to that of the Escherichia coli RecG protein, a junction-specific DNA helicase believed to be involved in the branch migration of recombinational intermediates. We have developed an expression system for the MmsA protein and have purified the MmsA protein to more than 99% homogeneity. The MmsA protein has DNA-dependent ATP hydrolysis and DNA junction-helicase activities that are similar to those of the E. coli RecG protein. The effect of the MmsA protein on the S. pneumoniae RecA protein-promoted three-strand exchange reaction was also investigated. In the standard direction (circular single-stranded (ss) DNA + linear double-stranded (ds) DNA → linear ssDNA + nicked circular dsDNA), the MmsA protein appears to promote the branch migration of partially exchanged intermediates in a direction opposite of the RecA protein, resulting in a nearly complete inhibition of the overall strand exchange reaction. In the reverse direction (linear ssDNA + nicked circular dsDNA → circular ssDNA + linear dsDNA), however, the MmsA protein appears to facilitate the conversion of partially exchanged intermediates into fully exchanged products, leading to a pronounced stimulation of the overall reaction. These results are discussed in terms of the molecular mechanism of transformational recombination.