Figure 2

Structure of the BLV provirus: genes, RNA transcripts and viral protein. The provirus is flanked by two identical long terminal repeat sequences (LTRs) and contains the open reading frames (orfs) corresponding to gag, prt (protease), pol and env. Several orfs coding for Tax, Rex, R3 and G4 are present in the X region between env and the 3'LTR. The genomic RNA transcript initiates and terminates in the 5' and 3' LTRs, respectively. This genomic RNA serves as a template for the expression of the gag-prt-pol precursors (pr145, pr66 and pr44) that are processed in structure and enzymatic proteins: matrix (MA) p15, capsid (CA) p24, nucleocapsid (NC) p12, protease (PRT) p14 and, p80 (RT/IN) harboring reverse transcriptase, RNAse H and integrase activities. A large intron corresponding to gag-prt-pol is excised to yield the envelope (env) RNA. After translation, the pr72 precursor is cleaved in two subunits: the extracellular (SU) gp51 and the transmembrane (TM) gp30 glycoproteins. To generate the Tax/Rex messenger RNA, a second intron is cleaved. This double-spliced RNA encodes both the p34 Tax protein using an initiation codon at the end of pol and Rex which shares the same AUG as Env pr72. Two minor RNAs identified by RT/PCR code for p5 (R3) and p11 (G4). The R3 RNA is similar to the double-spliced Tax/Rex message but the second intron is shorter and splicing occurs at the 5' end of the R3 frame. The R3 protein shares its aminoterminal end with Rex and pr72. In the G4 message, a very large intron is excised between a particular splice donor site different from that of the other viral RNAs and an acceptor just 5' to the G4 frame. The G4 protein initiates at a suboptimal CUG codon located in R of the 5'LTR.