HIV-1 RT is a heterodimeric protein.  The larger subunit, p66, is 560 amino acids long; the smaller subunit, p51, contains the first 440 amino acids of p66.  The p66 subunits folds into two domains, polymerase and RNase H.  The RNase H domain is shown in gold and the active site amino acids (Asp553, Glu478, Asp498, and Asp549) are shown on the linear diagram (upper left).  The polymerase domain is divided into four subdomains: fingers (blue), palm (red), thumb (green), and connection (yellow).  The active site amino acids (Asp110, Asp185, and Asp186) are located in the palm and are shown in the linear diagram.  The p51 subunit comprises the first 440 amino acids of p66, which corresponds closely, but not exactly, to the polymerase domain.  The p51 subunit folds into the same four subdomains as p66 (fingers, palm, thumb, and connection), but the relative arrangement of the subdomains is different in the two subunits.  The HIV-1 RT structure shown on the right corresponds to the structure of HIV-1 RT in a complex with DNA; however, to show the structure of RT more clearly, the DNA is not shown.