The PU.I site in the intron is important for the enhancer function of the EDN intron. (A) Schematic representation of the EDN promoter and intron. Putative transcription factor binding sites (CAAT box, AP1 sites, NFAT site, and PU.I site) are indicated as boxes. The arrows indicate the 3′ ends of the reporter constructs used in (B). ex1, exon 1. (B) The EDN reporter constructs described in (A) and the control vector pBLCAT3 were transfected into HL60 or HL60 7.7 cells as described in Fig 2. Construct +193 d PU.I contains point mutations in the PU.I site. The HL60 7.7 cells received BA 1 hour after transfection. CAT activity was determined 48 hours posttransfection. Bars indicate the mean percentage of acetylation of at least four independent experiments. The standard deviation is indicated by error bars. It is clear that the intron is essential for the activity of the EDN reporter construct, whereas the PU.I site seems to play a major role in the activity of the intron.