Protein isoforms encoded by the pX region of human T-cell leukemia/lymphotropic virus type I

The pX region of the human T-cell leukemia/lymphotropic virus type I (HTLV-I) contains at least four open reading frames (orf I-orf IV). orf III and orf IV encode the regulatory HTLV-I proteins Rex and Tax, which together modulate viral expression, and the p21(rex) protein of unknown function. By using the reverse transcriptase and polymerase chain reaction techniques on the RNA of an HTLV-I-infected cell culture, we uncovered the existence of alternatively spliced mRNAs generated through the use of three splice acceptor sites. These mRNAs encoded protein isoforms derived from the HTLV-I orf I (p12(I)) and orf II (p13(II) and p30(II)). An additional acceptor splice site, used in the processing of the env and tax/rex mRNAs and a singly spliced mRNA for the p21(rex) protein, was also identified. All of these HTLV-I mRNAs were also detected in freshly isolated cells from HTLV-I- infected individuals. Thus HTLV-I, like the human immunodeficiency virus type 1, has developed fine posttranscriptional mechanisms to increase the complexity of its genome.