DNA polymerase I enzymes have served as model systems to study the mechanis
m of template-directed DNA polymerization. This process requires that the e
nzyme cycles through a series of conformational changes, each cycle leading
to the incorporation of a nucleotide to the primer strand of the DNA. The
kinetics of nucleotide incorporation has been extensively studied leading t
o the definition of specific steps along the cycle. Efforts to visualize th
ese steps using X-ray crystallography have recently come to fruition, notab
ly for one particular DNA polymerase I system, that of Klentaql. This revie
w focuses on the structural characterization of the various steps along the
nucleotide incorporation pathway.