The mechanism by which DNA polymerase I enzymes function has been the subject of extensive biochemical and structural studies. We previously determined the structure of a ternary complex of the large fragment of DNA polymerase I from Thermus aquaticus (Klentaq1) bound to a primer/template DNA and a dideoxycytidine 5'-triphosphate (ddCTP). In this report, we present the details of the 2.3-A resolution crystal structures of three additional ternary complexes of Klentaq1 bound to a primer/template DNA and a dideoxyguanosine 5'-triphosphate (ddGTP), a dideoxythymidine 5'-triphosphate (ddTTP), or a dideoxyadenosine 5'-triphosphate (ddATP). Comparison of the active site of the four ternary complexes reveals that the protein residues around the nascent base pair (that formed between the incoming dideoxynucleoside triphosphate [ddNTP] and the template base) form a snug binding pocket into which only a correct Watson-Crick base pair can fit. Except in the ternary complex bound to dideoxyguanosine 5'-triphosphate, there are no sequence specific contacts between the protein side chains and the nascent base pair, suggesting that steric constraints imposed by the protein onto the nascent base pair is the major contributor to nucleotide selectivity at the polymerase active site. The protein around the polymerase active site also shows plasticity, which may be responsible for the substrate diversity of the enzyme. Two conserved side chains, Q754 and R573, form hydrogen bonds with the N3 atom in the purine base and O2 atom in the pyrimidine base at the minor groove side of the base pair formed by the incorporated ddNMP and the corresponding template base in all the four ternary complexes. These hydrogen-bonding interactions may provide a means of detecting misincorporation at this position.