ATTENUATION OF BOVINE LEUKEMIA-VIRUS BY DELETION OF R3 AND G4 OPEN READING FRAMES

Complex oncoviruses contain, in addition to the classical retroviral g enes (gag, pol, and env), a region (X) located between the envelope se quences and the 3' long terminal repeat. The X region contains two gen es, tar and rer, whose protein products are involved in transcriptiona l and posttranscriptional regulation of viral expression. In addition to these activators, the bovine leukemia virus (BLV) and the human T-c ell leukemia virus (HTLV) contain alternative open reading frames (R3 and G4 for BLV; p30, p13, and p12 for HTLV). As a virus/animal model f or HTLV-induced leukemogenesis, BLV provirus can be injected intraderm ally into sheep, where it induced B-lymphocyte transformation. Deletio n of the R3 and G4 sequences from an infectious and tumorigenic BLV pr ovirus greatly impaired the in vivo propagation of the viruses as demo nstrated by DNA polymerase chain reaction, RNA blots, structural-prote in ELISA, and immunofluorescence analysis. Our results show that the a lternative open reading frames are required for maintaining high virus loads during the course of persistent infection in vivo. Thus, R3 and G4 are candidates for antiviral drug development. Furthermore, viruse s with a deletion in these sequences should be tested as live attenuat ed vaccines.