Bovine leukemia virus: unique structural features of its long terminal repeats and its evolutionary relationship to human T-cell leukemia virus.

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Abstract
The nucleotide sequence of the long terminal repeat (LTR) of bovine leukemia virus, a unique oncogenic retrovirus of cattle, was determined. The LTR consisted of 530 base pairs (bp) with an inverted repeat of 6 bp at its 5'' and 3'' ends, flanked by a direct repeat of 6 bp of host cell origin. A tRNAPro binding site for minus-strand DNA synthesis followed the 5'' LTR. The U3 region contained putative transcriptional promoters, CAT box and TATA box, but they had peculiar sequences (C-C-A-A-C-T and G-A-T-A-A-A-T). The U3 region also contained a potential enhancer element, whose sequence partially resembled those of other viral and cellular, especially of Ig, enhancers. The most striking structural feature of the LTR was an exceptionally long R region (228 bp), which separated a poly(A) addition signal (A-A-T-A-A-A) from a poly(A) site as far apart as 260 bp. The long R region was suggested to form a large stable hairpin structure on a nascent RNA chain, making the 2 transcription termination signals close together and thus ensuring normal termination of the chain. This structural feature of the bovine leukemia virus LTR was analogous to that of human T-cell leukemia virus LTR and, in fact, slight sequence homology (at most 50%) was observed beween the R regions of these 2 retroviruses, indicating their evolutionary relationship. The unique structural feature of bovine leukemia virus and human T-cell leukemia virus LTR may thus bear some relation to the biological features commonly shared by these retroviruses.