Mapping the minimal ATRA-responsive FR-β promoter fragment. 293 cells were stably transfected with the different 5′ deleted FR-β promoter-luciferase reporter constructs as indicated; the numbers in parentheses indicate the nucleotide positions of the 5′ and 3′ ends of the FR-β promoter fragment relative to the transcription start site (+ 1 nt). The whole pool of recombinant 293 cells stably transfected with each FR-β promoter construct was treated with ATRA(1 μM, ▦) or the vehicle alone (□) for 3 days in 6-well plates. The cells were harvested and lysed at the end of the treatment, and the luciferase activity in the lysate was determined and described in “Materials and methods.” Panels A and B are from 2 representative experiments out of 6 experiments that were carried out identically. Each bar is expressed as means of triplicate experiments. Error bar stands for SD. Panel C shows the DNA sequence of the FR-β promoter fragment – 117ntto + 1 nt and the positions of the Sp1 and EBSs within it.