The HTLV-I Rex response element mediates a novel form of mRNA polyadenylation. 
HTLV-I structural gene expression is posttranscriptionally regulated by the Rex protein and the Rex response element (RexRE), a 255 nucleotide RNA stem-loop structure located in the retroviral 3' LTR.  Independent of Rex, the RexRE also plays a critical role in the polyadenylation of all HTLV-I transcripts.  Folding of the RexRE serves to spatially juxtapose the widely separated AAUAAA hexamer and GU-rich elements that are essential for polyadenylation.  In turn, this folding promotes the cooperative and stable binding of two nuclear factors at these elements that commits this poly(A) site to 3' processing.  These findings highlight a novel mechanism of 3' end formation in the HTLV family of retroviruses and underscore the general requirement for protein-protein interactions in the polyadenylation reaction.