Cooperative regulation of bovine leukaemia virus gene expression by two overlapping open reading frames in the XBL region.

Bovine leukaemia virus (BLV) induces syncytia in productively infected ovine and bovine monolayer cells. Expression of the env gene directly determines the syncytium-forming activity, since the expression of a cloned env gene directed by the simian virus 40 (SV40) early promoter efficiently induced syncytia in transfected ovine embryonic (OE) cells. However a BLV long terminal repeat (LTR)-directed expression plasmid (pLTRenv) failed to induce syncytia in transfected OE cells, suggesting insufficient promoter activity of the LTR sequences. To assess the role of the XBL genes, which are located in the 3' distal region of the genome, in viral gene expression we constructed SV40 early promoter-directed expression plasmids. These contained open reading frames (ORFs) in the XBL region, and were examined for syncytium-inducing activity by cotransfection with pLTRenv. The results suggest that both XBL-I (the longest ORF: x-lor) and XBL-II (a shorter overlapping ORF: x-sor) are trans-acting genes which cooperatively activate LTR-directed viral gene expression.