Complementation of the endonuclease and RT mutations. Assays were performed as diagrammed in Fig. 2A, by using the 110-bp end-labeled DNA target. Lanes: 1, no protein; 2, 4 ng PE..D mutation; 3, 4 ng PE..D and 4 ng YAYD mutations; and 4, 4 ng YAYD mutation. Neither the PE..D or YAYD mutant alone can use the DNA target to prime reverse transcription of the 283-nt R2 RNA; however, a mixture of the two proteins is capable. Twice the total level of protein was added in lane 3 compared with the wild-type lane in Fig. 2 to enable the formation of an equivalent amount of an active heterodimer of PE..D and YAYD compared with a wild-type dimer. We have no direct evidence, however, that such a heterodimer is formed.