FIG. 7.

USF1 is responsible for preventing chromosomal position-effect silencing. (A) Schematic representation of the insulated IL-2R transgene, which was amplified four times (four copies) in the 6C2 genome (line 809). (B) Line 809 was transfected with vector containing AUSF1 cDNA, which lacks the basic DNA-binding region and inhibits both USF1 and USF2 activities. The single clones (B7 and C11) were selected in the presence of 100 μg/ml zeocin (Invitrogen). Gel mobility shift assays of nuclear extracts from the control and AUSF1 clones were carried out using ³²P-labeled FIV oligonucleotide duplexes. (C) Results of the FACS analysis for IL-2R expression at 0 and 70 days after removal of hygromycin selection. M1 designates IL-2R-positive cells. On these histograms, cell number is indicated on the y axis and the logarithm of fluorescein isothiocyanate fluorescence intensity is on the x axis. (D) Results of the FACS analysis for IL-2R expression of clone B7 (top) and clone C11 (bottom) at 0 and 15 days after removal of hygromycin selection.