A superfamily of nucleic acid polymerases that includes the pol I and pol alpha classes of DNA-directed DNA polymerases, mitochondrial and phage DNA-directed RNA polymerases, and most RNA-directed polymerases may be defined on the basis of the occurrence of conserved sequence motifs and tertiary structure similarities between HIV-1 reverse transcriptase, DNA polymerase I and T7 RNA polymerase. Although sequence or structural similarities do not yet justify inclusion of the multi-subunit DNA-directed RNA polymerases in this superfamily, mechanistic similarities suggest a deep relationship between these and the simpler T7-like RNA polymerases.