Effect of Mg²⁺ on the DNA binding modes of the Streptococcus pneumoniae SsbA and SsbB proteins

The effect of Mg²⁺ on the binding of the Streptococcus pneumoniae single-stranded DNA binding (SSB) proteins, SsbA and SsbB, to various dT_n oligomers was examined by polyacrylamide gel electrophoresis. The results were then compared with those that were obtained with the well characterized SSB protein from Escherichia coli, SsbEc. In the absence of Mg²⁺, the results indicated that the SsbEc protein was able to bind to the dT_n oligomers in the SSB₃₅ mode, with only two of the four subunits of the tetramer interacting with the dT_n oligomers. In the presence of Mg²⁺, however, the results indicated that the SsbEc protein was bound to the dT_n oligomers in the SSB₆₅ mode, with all four subunits of the tetramer interacting with the dT_n oligomers. The SsbA protein behaved similarly to the SsbEc protein under all conditions, indicating that it undergoes Mg²⁺-dependent changes in its DNA binding modes that are analogous to those of the SsbEc protein. The SsbB protein, in contrast, appeared to bind to the dT_n oligomers in an SSB₆₅-like mode in either the presence or the absence of Mg ²⁺, suggesting that it may not exhibit the pronounced negative intrasubunit cooperativity in the absence of Mg²⁺ that is required for the formation of the SSB₃₅ mode. Additional experiments with a chimeric SsbA/B protein indicated that the structural determinants that govern the transitions between the different DNA binding modes may be contained within the N-terminal domains of the SSB proteins.