Thermodynamic analysis of DNA binding by a Bacillus single stranded DNA binding protein

We have cloned and purified SSB from Bacillus anthracis (SSBBA). In the absence of DNA, at concentrations ≤100 μg/ml, SSBBA did not form a stable tetramer and appeared to resemble bacteriophage T4 gene 32 protein. Fluorescence anisotropy studies demonstrated that SSBBA bound ssDNA with high affinity comparable to other prokaryotic SSBs. Thermodynamic analysis indicated both hydrophobic and ionic contributions to ssDNA binding. FRET analysis of oligo(dT)70 binding suggested that SSBBA forms a tetrameric assembly upon ssDNA binding. This report provides evidence of a bacterial SSB that utilizes a novel mechanism for DNA binding through the formation of a transient tetrameric structure.Unlike other prokaryotic SSB proteins, SSBBA from Bacillus anthracis appeared to be monomeric at concentrations ≤100 μg/ml as determined by SE-HPLC. SSBBA retained its ability to bind ssDNA with very high affinity, comparable to SSB proteins which are tetrameric. In the presence of a long ssDNA template, SSBBA appears to form a transient tetrameric structure. Its unique structure appears to be due to the cumulative effect of multiple key amino acid changes in its sequence during evolution, leading to perturbation of stable dimer and tetramer formation. The structural features of SSBBA could promote facile assembly and disassembly of the protein-DNA complex required in processes such as DNA replication.