BINDING OF DNA TO ALPHA-BETA-TYPE SMALL, ACID-SOLUBLE PROTEINS FROM SPORES OF BACILLUS OR CLOSTRIDIUM SPECIES PREVENTS FORMATION OF CYTOSINE DIMERS, CYTOSINE-THYMINE DIMERS, AND BIPYRIMIDINE PHOTOADDUCTS AFTER UV IRRADIATION

Small, acid-soluble proteins (SASP) of the alpha/beta-type from spores of Bacillus and Clostridium species bind to DNA; this binding prevents formation of cyclobutane-type thymine dimers upon UV irradiation, but promotes formation of the spore photoproduct, an adduct between adjacent thymine residues. alpha/beta-Type SASP also bound to poly(dG) . poly(dC) and poly(dA-dG) . poly(dC-dT). While UV irradiation of poly(dG) . poly(dC) produced cyclobutane-type cytosine dimers as well as fluorescent bipyrimidine adducts, the yields of both types of photoproduct were greatly reduced upon irradiation of alpha/beta-type SASP-poly(dG) . poly(dC) complexes. UV irradiation of poly(dA-dG) . poly(dC-dT) produced a significant amount of a cyclobutane dimer between cytosine and thymine, as well as a 6-4 bipyrimidine adduct. Again, binding of alpha/beta-type SASP to poly(dA-dG) . poly(dC-dT) greatly reduced formation of these two photoproducts, although formation of the cytosine-thymine analog of the spore photoproduct was not observed. These data provide further evidence for the dramatic change in DNA structure and photoreactivity which takes place on binding of alpha/beta-type SASP and suggest that binding of these proteins to DNA in vivo prevents formation of most deleterious photoproducts upon UV irradiation.