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

Diane E Grove1, Floyd R Bryant

  • 1Department of Biochemistry and Molecular Biology, Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, MD 21205, USA.

The Journal of Biological Chemistry
|November 22, 2005
PubMed

Insights

Magnesium ions (Mg2+) influence how Streptococcus pneumoniae single-stranded DNA binding (SSB) proteins SsbA and SsbB interact with DNA. SsbA shows Mg2+-dependent binding, while SsbB binds similarly regardless of Mg2+ presence.

Area of Science:

  • Molecular Biology
  • Biochemistry
  • Microbiology

Background:

  • Single-stranded DNA binding (SSB) proteins are crucial for DNA replication, repair, and recombination in bacteria.
  • The Streptococcus pneumoniae SSB proteins, SsbA and SsbB, play vital roles in maintaining genomic stability.
  • Magnesium ions (Mg2+) are known to modulate protein-DNA interactions in various biological systems.

Purpose of the Study:

  • To investigate the effect of Mg2+ on the DNA binding modes of Streptococcus pneumoniae SsbA and SsbB proteins.
  • To compare the Mg2+-dependent DNA binding behavior of S. pneumoniae SSB proteins with the well-characterized Escherichia coli SSB (SsbEc).
  • To identify potential structural determinants responsible for Mg2+-dependent DNA binding transitions.

Main Methods:

  • Polyacrylamide gel electrophoresis (PAGE) was employed to analyze the binding of SSB proteins to dT(n) oligomers.
  • Experiments were conducted in the presence and absence of Mg2+ to assess its influence on binding.
  • Comparative analysis was performed between S. pneumoniae SSB proteins (SsbA, SsbB) and E. coli SSB (SsbEc).

Main Results:

  • SsbEc and SsbA exhibit Mg2+-dependent DNA binding, transitioning between SSB(35) and SSB(65) modes.
  • In the absence of Mg2+, SsbEc binds in SSB(35) mode (two subunits interact); in its presence, it shifts to SSB(65) mode (all four subunits interact).
  • SsbB consistently binds in an SSB(65)-like mode, irrespective of Mg2+ presence, suggesting altered cooperativity.
  • A chimeric SsbA/B protein study indicated that N-terminal domains likely contain structural elements governing these binding mode transitions.

Conclusions:

  • SsbA displays Mg2+-dependent DNA binding modes analogous to SsbEc, highlighting conserved regulatory mechanisms.
  • SsbB's Mg2+-independent binding suggests a distinct mechanism, potentially lacking the negative cooperativity required for the SSB(35) mode.
  • The N-terminal domains of SSB proteins are critical for mediating Mg2+-dependent alterations in DNA binding modes.

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