Molecular analysis of a deletion mutant provirus of type I human T-cell lymphotropic virus: evidence for a doubly spliced x-lor mRNA.

The genome of the human T-cell leukemia/lymphotropic virus type I (HTLV-I) contains a functional gene denominated x-lor that may be important in HTLV-I transformation of human T cells. To study the role of x-lor and other HTLV-I genes in cellular transformation, we obtained a transformed nonproducer human T-cell line containing a single defective HTLV-I provirus (HTLV-I 55/PL). This 7-kilobase provirus had undergone a deletion involving the entire envelope gene and the nonconserved region. The point of the deletion corresponded to the junction of a donor splice site, located between the polymerase gene and the envelope gene (nucleotide 5183), and the acceptor site for the mRNA of the x-lor gene (nucleotide 7302). The juxtaposition of nucleotides 5182 and 7302 brings the initiating methionine codon of the envelope gene immediately 5' to the x-lor region, leaving the DNA sequence in frame for expression of a protein product. This finding suggests that a double splicing mechanism is used to express the x-lor gene, and that the defective provirus 55/PL was generated through the reverse transcription of a partially spliced mRNA. Analysis of the x-lor mRNA of other HTLV-I-transformed cell lines revealed that a double splicing process is commonly used. Furthermore, since 55/PL can be faithfully transmitted and is able to immortalize recipient T cells, we can conclude that the envelope gene is not necessary for in vitro transformation by HTLV-I.