Equilibrium and kinetic binding interactions between DNA and a group of novel, nonspecific DNA-binding proteins from spores of Bacillus and Clostridium species

Binding of alpha/beta -type small acid-soluble spore proteins (SASP) is the major determinant of DNA resistance to damage caused by UV radiation, heat , and oxidizing agents in spores of Bacillus and Clostridium species. Analy sis of several alpha/beta -type SASP showed that these proteins have essent ially no secondary structure in the absence of DNA, but become significantl y cu-helical upon binding to double-stranded DNAs or oligonucleotides. Fold ing of alpha/beta -type SASP induced by a variety of DNAs and :oligonucleot ides was measured by CD spectroscopy, and this allowed determination of a D NA binding site size of 4 base pairs as well as equilibrium binding paramet ers of the alpha/beta -type SASP-DNA interaction, Analysis of the equilibri um binding data further allowed determination of both intrinsic binding con stants (LC) and cooperativity factors (omega), as the alpha/beta -type SASP -DNA interaction was significantly cooperative, with the degree of cooperat ivity depending on both the bound DNA and the salt concentration, Kinetic a nalysis of the interaction of one alpha/beta -type SASP, SspC(Tyr), With DN A indicated that each binding event involves the dimerization of SspC(Tyr) monomers at a DNA binding site. The implications of these findings for the structure of the alpha/beta -type SASP.DNA complex and the physiology of al pha/beta -type SASP degradation during spore germination are discussed.