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The pX sequence of human T-cell leukemia virus type I codes for two nuclear proteins, p40tax and p27rex, and a cytoplasmic protein, p21x-III.p40tax activates transcription from the long terminal repeat (LTR), whereas p27rex modulates posttranscriptional processing to accumulate gag and env mRNAs that retain intron sequences. In this paper, we identify two cis-acting sequence elements needed for regulation by p27rex: a 5' splice signal and a specific sequence in the 3' LTR. These two sequence elements are sufficient for regulation by p27rex; expression of a cellular gene (metallothionein I) became sensitive to rex regulation when the LTR was inserted at the 3' end of this gene. The requirement for these two elements suggests an unusual regulatory mechanism of RNA processing in the nucleus.
Seiki et al. (Sat,) studied this question.