A novel alternatively spliced viral mRNA transcribed in cells infected with human T cell leukemia virus type 1 is mainly responsible for expressing p21X protein.

The pX sequence of human T cell leukemia virus type 1 (HTLV-1) has been thought to be expressed as a doubly spliced mRNA that codes for p40tax, p27rex and p21X. However, we identified a novel alternatively spliced mRNA in the HTLV-1 infected cells by using reverse transcription followed by the polymerase chain reaction. This mRNA contains only the first and third exons of the doubly spliced mRNA and encodes only p21X. Our data that this mRNA is responsible for expressing p21X exists in most of HTLV-1 infected cells strongly suggests that p21X may play a crucial role for HTLV-1 replication.