Effect of Mg super(2+) on the DNA Binding Modes of the Streptococcus pneumoniae SsbA and SsbB Proteins

• Published in: The Journal of biological chemistry 2006-01, Vol.281 (4), p.2087-2094
• Main Authors: Grove, Diane E, Bryant, Floyd R
• Format: Article
• Language: English
• Subjects: Escherichia coli; Streptococcus pneumoniae
• ISSN: 0021-9258; 1083-351X

The effect of Mg super(2+) on the binding of the Streptococcus pneumoniae single-stranded DNA binding (SSB) proteins, SsbA and SsbB, to various dT sub(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 Mg super(2+), the results indicated that the SsbEc protein was able to bind to the dT sub(n) oligomers in the SSB sub(35) mode, with only two of the four subunits of the tetramer interacting with the dT sub(n) oligomers. In the presence of Mg super(2+), however, the results indicated that the SsbEc protein was bound to the dT sub(n) oligomers in the SSB sub(65) mode, with all four subunits of the tetramer interacting with the dT sub(n) oligomers. The SsbA protein behaved similarly to the SsbEc protein under all conditions, indicating that it undergoes Mg super(2+)-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 sub(n) oligomers in an SSB sub(65)-like mode in either the presence or the absence of Mg super(2+), suggesting that it may not exhibit the pronounced negative intrasubunit cooperativity in the absence of Mg super(2+) that is required for the formation of the SSB sub(35) 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.