THE HYPERTHERMOPHILE CHROMOSOMAL PROTEIN SAC7D SHARPLY KINKS DNA

The proteins Sac7d and Sso7d belong to a class of small chromosomal pr oteins from the hyperthermophilic archaeon Sulfolobus acidocaldarius a nd S. solfactaricus, respectively(1,2). These proteins are extremely s table to heat, acid and chemical agents', Sac7d binds to DNA without a ny particular sequence preference and thereby increases its melting te mperature by similar to 40 degrees C (ref. 4). We have now solved and refined the crystal structure of Sac7d in complex with two DNA sequenc es to high resolution, The structures are examples of a nonspecific DN A-binding protein bound to DNA, and reveal that Sac7d binds in the min or groove, causing a sharp kinking of the DNA helix that is more marke d than that induced by an): sequence-specific DNA-binding proteins, Th e kink results from the intercalation of specific hydrophobic side cha ins of Sac7d into the DNA structure, but without causing any significa nt distortion of the protein structure relative to the uncomplexed pro tein in solution.