Trans-activation of the MDR1 gene promoter by the HTLV-I tax protein. (A) Representative experiment showing chloramphenicol acetyl transferase (CAT) assays of COS cell extracts. (B) Quantification of CAT activity in COS cell extracts from three independent experiments. The percentage of chloramphenicol converted into acetylated forms is shown. COS cells were electroporated, as described in the text, with the following plasmids: 1: No DNA; 2: pBC-tax/rex; 3: pBC-rex; 4: pRK7-tax; 5: pMDR(+)CAT; 6: pMDR(-)CAT; 7: pMDR-CAT (M.M. Gottesman); 8: pLTR-I-CAT; 9: pMDR(+)CAT + pBC-tax/rex; 10: pMDR(-)CAT + pBC-tax/rex; 11: pMDR-CAT (M.M. Gottesman) + pBC-tax/rex; 12: pLTR-I-CAT + pBC-tax/rex; 13: pMDR(+)CAT + pBC-rex; 14: pMDR(-)CAT + pBC-rex; 15: pMDR-CAT (M.M. Gottesman) + pBC-rex; 16: pLTR-CAT + pBC-rex; 17: pMDR(+)CAT + pRK7-tax; 18 pMDR(-)CAT + pRK7-tax; 19: pMDR-CAT (M.M. Gottesman) + pRK7-tax; 20: pLTR-I-CAT + pRK7-tax. Therefore, lanes 1 to 8, 10, 14, and 18 are negative controls; lanes 12 and 20 are positive controls. Trans-activation of the MDR1 promoter-CAT constructs by tax can be seen in lanes 9, 11, 17, and 19. Lanes 13 to 16 show negligible activation of the MDR1 promoter-CAT constructs by rex.