Construction and structure of recombinant plasmids pBC-tax/rex, pBC-rex, and pMDR-CAT. (A) RT-PCR amplification from HTLV tax/rex mRNA was used to generate a 1,098-bp fragment, which encodes the complete tax, rex, and p21 proteins. (B) The tax/rex protein expression construct, pBC-tax/rex, was derived by insertion of the 1,098-bp tax/rex cDNA into pBC12/CMV/IL-2 under the control of the CMV immediate early (CMV IE) promoter. The rex protein expression construct, pBC-rex, was derived by deletion of an AvaI-Ava I fragment in pBC-tax/rex, which removes the C-terminal half of tax. (C) MDR1 promoter-CAT reporter gene constructs. pMDR(+)CAT contains the MDR1 promoter in tandem with the CAT gene. pMDR(-)CAT contains the MDR1 promoter in a reverse orientation to the CAT gene.