Differences in Unfolding Energetics of CcdB Toxins From V. fischeri and E. coli

Ccd system is a toxin-antitoxin module (operon) located on plasmids and chromosomes of bacteria. CcdBF encoded by ccd operon located on Escherichia coli plasmid F and CcdBVfi encoded by ccd operon located on Vibrio fischeri chromosome are members of the CcdB family of toxins. Native CcdBs are dimers that bind to gyrase-DNA complexes and inhibit DNA transcription and replication. While thermodynamic stability and unfolding characteristics of the plasmidic CcdBF in denaturant solutions are reported in detail, the corresponding information on the chromosomal CcdBVfi is rather scarce. Therefore, we studied urea-induced unfolding of CcdBVfi at various temperatures and protein concentrations by circular dichroism spectroscopy. Global model analysis of spectroscopic data suggests that CcdBVfi dimer unfolds to the corresponding monomeric components in a reversible two-state manner. Results reveal that at physiological temperatures CcdBVfi exhibits lower thermodynamic stability compared to CcdBF. At high urea concentrations CcdBVfi, similarly to CcdBF, retains a significant amount of secondary structure. Differences in thermodynamic parameters of CcdBVfi and CcdBF unfolding can reasonably be explained by the differences in their structural features.