A, Schematic diagram of wild type and Rev mutants. The location of the M4 mutations are indicated by arrows. The Rev basic domain is indicated as Rev-NOS, the three copies of the large T-antigen NLS are indicated as 3xNLS, the Rex overlapping NLS/NOS signal is shown as Rex-NOS. B, Schematic diagram of the reporter systems. The CAT gene and the 5' and 3' splice sites are indicated. The Rev and Rex responsive elements are indicated as RRE and RxRE respectively. The 6 copies of the Rev high affinity binding site IIB is indicated as 6xIIB. The Rev dependent gag mRNA with the CRS element fused to the RRE expressed in the A3.9 cells is shown below. The drawings are not to scale.