DNA and RNA polymerase exhibit similarities in structures and catalytic mechanisms, suggesting that both classes of enzymes are evolutionarily related. To probe the biochemical and structure-function relationship between the two classes of polymerases, a large library (200,000 members) of mutant Thermus aquaticus DNA polymerase I (Taq pol I) was created containing random substitutions within a portion of the dNTP binding site (motif A; amino acids 605-617), and a fraction of all selected active Taq pol I (291 of 8000) was tested for the ability to incorporate successive ribonucleotides; 23 unique mutants that added rNTPs into a growing polynucleotide chain were identified and sequenced. These mutants, each containing one to four substitutions, incorporate ribonucleotides at a efficiency approaching 10(3)-fold greater than that of wild type Taq pol I. Several mutants added successive ribonucleotides and thus can catalyze the synthesis of RNA. Sequence analysis of these mutants demonstrates that at least two amino acid residues are involved in excluding ribonucleotides from the active site. Interestingly, wild type DNA polymerases from several distinct families selectively discriminate against rUTP. This study suggests that current DNA and RNA polymerases could have evolved by divergent evolution from an ancestor that shared a common mechanism for polynucleotide synthesis.