Multiple sequence elements are required for regulation of human T-cell leukemia virus gene expression.

The U3 region of the long terminal repeat (LTR) of human T-cell leukemia virus type I (HTLV-I) contains sequences that respond to the trans-activating transcription (tat) factor encoded by the pX region of the provirus. Results presented here show that there are multiple tat-responsive sequences within the LTR and that a single 21-nucleotide sequence, which is repeated three times within the U3 region, is sufficient to determine the response to the trans-activator. This sequence is capable of conferring a tat-responsive phenotype upon the HTLV-I and simian virus 40 promoters, independent of orientation. Sequences required for efficient HTLV-I LTR-directed gene expression are also located 3' to the site of RNA initiation, within the R and U5 regions of the LTR.