A 15-residue sequence motif has been found in many polymerases from various species and involving DNA and RNA dependence and product. The motif is characterized by a Tyr-Gly-Asp-(Thr)-Asp core flanked by hydrophobic spans five residues in length. An mRNA maturase segment is also suggested to display the motif pattern. The aspartates may be important in polymerase function by acting directly in catalysis and/or by binding magnesium.