The analysis of the deduced amino acid sequence of the herpes simplex virus type 1 (HSV-1) DNA polymerase reported here suggests that the polymerase structure consists of domains carrying separate biological functions. The HSV-1 enzyme is known to possess 5'-3'-exonuclease (RNase H), 3'-5'-exonuclease, and DNA polymerase catalytic activities. Sequence analysis suggests an arrangement of these activities into distinct domains resembling the organization of Escherichia coli polymerase I. In order to more precisely define the structure and C-terminal limits of a putative catalytic domain responsible for the DNA polymerization activity of the HSV-1 enzyme, we have undertaken in vitro mutagenesis and computer modeling studies of the HSV-1 DNA polymerase gene. Sequence analysis predicts that the major DNA polymerization domain of the HSV-1 enzyme will be contained between residues 690 and 1100, and we present a three-dimensional model of this region, on the basis of the X-ray crystallographic structure of the E. coli polymerase I. Consistent with these structural and modeling studies, deletion analysis by in vitro mutagenesis of the HSV-1 DNA polymerase gene expressed in Saccharomyces cerevisiae has confirmed that certain amino acids from the C terminus (residues 1073 to 1144 and 1177 to 1235) can be deleted without destroying HSV-1 DNA polymerase catalytic activity and that the extreme N-terminal 227 residues are also not required for this activity.