Crystal structures of open and closed forms of binary and ternary complexes of the large fragment of Thermus aquaticus DNA polymerase I: structural basis for nucleotide incorporation

The crystal structures of two ternary complexes of the large fragment of Th ermus aquaticus DNA polymerase I (Klentaq1) with a primer/template DNA and dideoxy-cytidine triphosphate, and that of a binary complex of the same enz yme with a primer/template DNA, were determined to a resolution of 2.3, 2.3 and 2.5 Angstrom, respectively. One ternary complex structure differs mark edly from the two other structures by a large reorientation of the tip of t he fingers domain. This structure, designated 'closed', represents the tern ary polymerase complex caught in the act of incorporating a nucleotide, In the two other structures, the tip of the fingers domain is rotated outward by 46 degrees ('open') in an orientation similar to that of the apo form of Klentaq1, These structures provide the first direct evidence in DNA polyme rase I enzymes of a large conformational change responsible for assembling an active ternary complex.