Protein-nucleotide interactions in E-coli DNA topoisomerase I

DNA topoisomerases are the enzymes responsible for controlling and maintain ing the topological states of DNA. Type IA enzymes work by transiently brea king the phosphodiester backbone of one strand to allow passage of another strand through the break. The protein has to perform complex rearrangements of the DNA, and hence it is likely that different regions of the enzyme bi nd DNA with different affinities. In order to identify some of the DNA bind ing sites in the protein, we have solved the structures of several complexe s of the 67 kDa N-terminal fragment of Escherichia coli DNA topoisomerase I with mono- and trinucleotides. There are five different binding sites in t he complexes, one of which is adjacent to the active site. Two other sites are in the central hole of the protein and may represent general DNA bindin g regions. The positions of these sites allow us to identify different DNA binding regions and to understand their possible roles in the catalytic cyc le.