A single species of pX mRNA of human T-cell leukemia virus type I encodes trans-activator p40x and two other phosphoproteins.

Human T-cell leukemia virus type I (HTLV-I) contains the pX sequence which codes for the trans-activator of the long terminal repeat (LTR) and is thus postulated to be associated with leukemogenesis in adult T-cell leukemia. Overlapping open reading frames (ORF) in the pX sequence were recently found to code for p27x-III and p21x-III by ORF III, in addition to p40x coded for by ORF IV. The mechanism of expression of these newly identified proteins and their possible association with trans-activation were studied. On transfection of an expression plasmid that contains a cDNA sequence of the pX mRNA, products from both ORFs III and IV were detected in the cells. The RNA was synthesized in vitro from the cDNA clone by SP6 RNA polymerase and translated in a rabbit reticulocyte lysate. As translation products, two proteins, p27x-III and p21x-III, were detected in addition to p40x. Elimination of the first and second ATG codons in ORF III resulted in loss of the ability to code for p27x-III and p21x-III, respectively, which indicated that the translations from these two ATG codons were independent. A mutant that lacked both ATG codons was fully active in trans-activation of chloramphenicol acetyltransferase gene expression directed by the LTR. These results indicate that a 2.1-kilobase pX mRNA of HTLV-I independently encodes three proteins, p40x, p27x-III, and p21x-III, by different ORFs and that the last two proteins are not involved in trans-activation of the unintegrated LTR.