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The mouse mammary tumor virus (MMTV) encodes a 5' element crucial for transcription of its genome along with the Rem/Rem-responsive element (RmRE) responsible for nuclear export of this unspliced RNA. Whether the 5' element is Rem-responsive or has any functional interaction with host/viral factors to facilitate MMTV gene expression was tested in this study. Our results reveal that the 5' element is non-responsive to Rem, but can be transactivated by both HIV Tat and HTLV-1 Tax activators. Reciprocally, MMTV could transactivate not only HIV TAR (similar to HTLV Tax), but also its 5' element. Furthermore, we reveal involvement of pTEFb, a general elongation factor associated with transactivation by Tat/Tax. This makes MMTV the first betaretrovirus to encode both Rem/RRE and Tat/TAR-Tax/TRE-like transcription regulatory systems. This study should enhance not only our understanding of retrovirus replication and virally-induced cancers/immunodeficiency syndromes, but also development of improved retroviral vectors for human gene therapy. The mouse mammary tumor virus (MMTV) encodes a gene transactivation system similar to HIV Tat/TAR-HTLV Tax/TRE that requires a yet-to-be identified virally-encoded factor and the cellular factor pTEF-b for function. [ABSTRACT FROM AUTHOR]