Molecular characterization of a variant of proviral bovine leukaemia virus (BLV).

Southern-blot hybridization and partial sequencing of the pol and env genes were used to characterize BLV-integrated provirus of seropositive cattle from two dairy herds in northern Italy. Comparison of the data obtained with those of previously characterized BLV strains from other geographic areas (Australia, Belgium, Japan and USA) revealed the presence of a viral variant (BLV-12), which showed both conserved and unique features. Regarding the gp51 envelope glycoprotein, the BLV-12 variant showed: 1. A high extent of conservation, which included potential glycosylation sites and cysteine residues; 2. Three unique amino acid residues not present in any of the other BLV strains analysed; and 3. Some variability at the level of one (G) of the three (F, G and H) conformational epitopes, which is probably important in the process of infection. These results agree with the suggestion that the sequence variability of the gp51 glycoprotein preferentially involves structures whose change is thought to underlie the phenomenon of escape from immune surveillance.