Complete nucleotide sequence of the genome of bovine leukemia virus: its evolutionary relationship to other retroviruses.

We report the complete 8714-nucleotide sequence of the integrated bovine leukemia virus genome and deduce the following genomic organization: 5' LTR-gag-pol-env-pXBL-3' LTR, where LTR represents a long terminal repeat and pXBL represents a region containing unidentified open reading frames. This genomic structure is similar to that of human T-cell leukemia virus. The LTR contains a putative splice donor site in the R region. The gag gene encodes a precursor protein with the form NH2-p15-p24-p12-COOH. The NH2- and COOH-terminal regions of the pol product show stronger homologies with those of avian, rather than murine, type C retrovirus, and its structure is identical to that of avian virus. The env gene encodes a surface glycoprotein (gp51) and a transmembrane protein (gp30). In contrast to the pol product, the gp30 shows stronger sequence homology with a murine, rather than avian homologue, indicating the chimeric nature of the bovine leukemia virus genome. Comparisons of the best conserved pol sequences and overall genomic organizations between several major oncoviruses allow us to propose that bovine leukemia and human T-cell leukemia viruses constitute a group, designated as type "E," of Oncovirinae.