STABILITY OF ESCHERICHIA-COLI SINGLE-STRANDED-DNA BINDING-PROTEIN (ECOSSB)

Conformation and stability of EcoSSB, a single-stranded DNA binding pr otein encoded by Escherichia coli, were analyzed by circular dichroism and fluorescence measurements. From CD measurements at pH 7.5, EcoSSB can be classified as a protein with high alpha-helix and beta-sheet c ontent. The hydrophobicity of the environment of the tryptophan residu es of the native protein is only marginally increased in comparison to the unfolded protein. The GdnHCl induced unfolding curves measured by CD and fluorescence are coincident and sigmoidal and show a monophasi c transition. The stability of EcoSSB is concentration dependent and t he unfolding behavior can be described as a two-state transition from the folded tetrameter to unfolded monomers. The mean values of free en ergy of dissociation and unfolding Delta G(u)(H2O) are between 173 and 177 kJ . mol(-1) and the mean half concentration c(1/2) of GdnHCl of the transition curves are about 1.5 M and 1.7 M for protein concentrat ions of 0.1 mg . ml(-1) and 0.5 mg . ml(-1), respectively.