The U5 sequence is a cis-acting repressive element for genomic RNA expression of human T cell leukemia virus type I.

The long terminal repeat (LTR) of HTLV-1 consists of U3-R-U5 and contains elements which regulate the viral gene expression. In addition to these elements, we found that the U5 sequence contained a repressive element in the gene expression. Sequential deletions of the LTR substantially increased CAT expression of a LTR-CAT construct. The repressive effect was also observed at RNA level on unspliced RNA, but not on spliced RNA. These differential effects suggest that the U5 element is a cis-acting RNA element, since splicing within the R segment removed the U5 element. In the context of HTLV-1 gene expression, the U5 element would repress expression of unspliced genomic RNA of HTLV-1, but not subgenomic RNA.