Repetitive structure in the long-terminal-repeat element of a type II human T-cell leukemia virus.

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Abstract
The majority of human T-cell leukemia virus isolates (HTLV-I) are associated with clinically aggressive adult T-cell leukemia/lymphomas. HTLV-II was isolated from a patient with a relatively benign hairy T-cell leukemia. To characterize differences in the viral genomes that might contribute to these different pathologies, the nucleotide sequence of the long terminal repeat (LTR) of a HTLV-II provirus was determined. Comparison with the type I HTLV LTR reveals that, whereas the overall structural features are similar, the 2 sequences differ markedly throughout most of the length of the LTR. Despite the overall differences, the sequences of several functional regions of the 2 LTR are conserved. These include the 5'' boundary of U3, the RNA cap site, and the tRNAPro-binding site immediately 3'' to the LTR. Another point of similarity is a 21-base sequence that is repeated 4 times in the U3 region of HTLV-II and 3 times in the U3 region of HTLV-I. This sequence has a formal analogy to, but no common sequence with, viral transcriptional enhancers. The U3 region of HTLV-II possesses a series of imperfect tandem direct repeats, 42 bases long, 21 bases long, 19 bases long and 7 bases long. These structures differ from those of HTLV-I except for the 21-base repeat sequence. Thus, the structure of HTLV-II differs substantially from that of HTLV-I in the region that governs transcriptional initiation and tissue specificity. Such differences may account for some of the differences in clinical presentation of HTLV-associated adult T-cell leukemia/lymphomas and hairy T-cell leukemia.