Ribbon diagram of the HIV-1 RT bound to a space-filling model of an 18–19-base duplex oligonucleotide substrate. The regions of A-form, B-form, and bent DNAs are indicated. The various domains of the enzyme are color-coded as in Fig. 4. The active site amino acids of the DNA polymerase and RNase H (which are both present in the p66 subunit) are shown in white. (Reprinted, with permission, from Jacobo-Molina et al. 1993.)