A MGF STAT5 BINDING-SITE IS NECESSARY IN THE DISTAL ENHANCER FOR HIGHPROLACTIN INDUCTION OF TRANSFECTED RABBIT ALPHA-S1-CASEIN-CAT GENE-TRANSCRIPTION/

The rabbit alpha s1-casein gene contains a distal prolactin-dependent enhancer 3442-3285 bp 5' to the site of initiation of transcription, W e have reported previously that four DNA/protein-binding sites (F1-F4) are located within this distal enhancer, We now show that one of this binding site (the F4 site) binds in vitro a MGF/STAT5-like factor, Th e functional importance of the F4 site was estimated by cotransfection of CHO cells with a chimeric gene containing or not the F4 sequence l inked to the (-391/+1774)CAT gene and a plasmid encoding the rabbit ma mmary prolactin receptor, The F4 site is necessary for maximal respons e of the enhancer to prolactin, However, this site has to be associate d to the F1-F3 fragment, It can be replaced by a genuine MGF/STAT5-bin ding site, A mutational analysis indicates that F4 and F1 sites are si multaneously involved to confer a high prolactin sensitivity.