The human immunodeficiency virus type 1 polyadenylylation signal: a 3' long terminal repeat element upstream of the AAUAAA necessary for efficient polyadenylylation.

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Abstract
Several polyadenylylation (PA) signals containing elements upstream of the AAUAAA have recently been characterized. Similar to PA elements found downstream of the AAUAAA, the upstream elements function to increase efficiency of AAUAAA use as a signal for cleavage and PA. Using deletion and linker scanning mutations we show that the PA signal of human immunodeficiency virus type 1 contains upstream elements transcribed from the U3 region of the 3' long terminal repeat. The element that has the greatest effect on PA site use lies 77 to 94 nucleotides upstream of the AAUAAA, between the TATA element and the transcriptional initiation site. Mutations in the adjacent region, between 59 and 76 nucleotides upstream of the AAUAAA, have a smaller effect on PA efficiency. Mutations in a region further upstream, between 141 and 176 nucleotides upstream of the AAUAAA, also affected PA modestly. Functional similarity between upstream elements was indicated by the ability of the human immunodeficiency virus upstream region to replace the upstream region of the simian virus 40 late PA signal. The sequence of the major upstream element of human immunodeficiency virus is uracil-rich, analogous to many defined downstream PA elements. This fact may imply that upstream and downstream elements have similar mechanisms of action.