FIG. 1.

Construction and characterization of mutant MMTV LTRs. (A) Diagram of the hybrid proviral clone of MMTV, HYB-MTV. The 5′ portion of the provirus, including the LTR, is derived from the endogenous MMTV strain Mtv1 (white), while the 3′ portion, including the LTR, is derived from the exogenous C3H MMTV (gray). The coiled extension at the 3′ end of the provirus denotes the flanking cellular DNA at the site of proviral integration. The recognition sites of restriction enzymes used in this study are also shown. (B) Diagram of the MMTV, TBLV, and mutant MMTV LTRs. The arrow on the MMTV LTR shows the transcription start site at the U₃-R border. sag coding potential is shown as a black (full-length Sag) or gray (truncated Sag) box under each LTR. (C) Transcriptional efficiencies of wild-type MMTV, mutant MMTV, and TBLV LTRs in transient-transfection assays of Jurkat T cells. Luciferase (LUC) activity is reported in light units normalized for DNA uptake as measured by cotransfection with the Renilla luciferase expression plasmid, pRL-TK. LUC activities from the TBLV and mutant MMTV LTRs are depicted relative to that from the MMTV LTR (assigned a value of 1). Standard deviations from the means of triplicate assays are shown. (D) Transcriptional efficiencies of wild-type MMTV, mutant MMTV, and TBLV LTRs in HC11 mouse mammary cells. HC11 cells were grown in the absence (white bars) or presence (gray bars) of 10⁻⁶ M DEX for 24 h prior to assays. LUC activity is reported in light units normalized for DNA uptake as measured by cotransfection with the pRL-TK reporter plasmid. LUC activities from the TBLV and mutant MMTV LTRs are depicted relative to that from the MMTV LTR in the absence of DEX (assigned a value of 1). Standard deviations from the means of triplicate assays are shown. Significance was determined by Student's t test.