Figure 3.4.

Development of the packaging cassette. Four generations of the packaging cassette are shown. (A) First generation included all four accessory proteins, Vpu, Vpr, Vif, and Nef and the regulatory proteins, tat and rev. RRE stands for rev response element. Expression is driven from the CMV promoter; PolyA signal (pA) is shown. (B) Second generation excluded all four accessory proteins, Vpu, Vpr, Vif, and Nef, but included regulatory proteins, tat and rev. (C) Further split of the packaging cassette defined the third generation. (D) Four generation characterized by a further split of the cassette into three components: gag expressed from the CMV promoter; Vpr-protease expressed from the EF1-α promoter. Vpr-pol transcript is driven from the EF1-α promoter. All three packaging cassettes contain RRE and pA signals. CMV, cytomegalovirus; RRE, Rev response element.