The PRD1 DNA polymerase is a small multi-functional enzyme containing conserved amino acid sequences shared by family B DNA polymerases. Thus the PRD1 DNA polymerase provides an useful model system with which to study structure-functional relationships of DNA polymerase molecules. In order to investigate the functional and structural roles of the highly conserved amino acid sequences, we have introduced three mutations into a conserved amino acid of the PRD1 DNA polymerase. Genetic complememtation study indicated that each mutation inactivated DNA polymerase catalytic activity.