THE RAT GROWTH-HORMONE AND HUMAN CELLULAR RETINOL-BINDING PROTEIN-1 GENES SHARE HOMOLOGOUS NF1-LIKE BINDING-SITES THAT EXERT EITHER POSITIVE OR NEGATIVE INFLUENCES ON GENE-EXPRESSION IN-VITRO

High levels of expression for the rat growth hormone (rGH) gene are re stricted to the somatotroph cells of the anterior pituitary, Previousl y, we have shown that rGH cell-specific repression results in part fro m the recognition of negatively acting silencers by a number of nuclea r proteins that repress basal promoter activity, Examination of these silencers revealed the presence of binding sites for proteins that bel ong to the NF1 family of transcription factors, Indeed, proteins from this family were shown to bind the rGH proximal silencer (designated s ilencer-1) in in vitro assays, Furthermore, this silencer site is capa ble of repressing chloramphenicol acetyltransferase (CAT) gene express ion driven by an heterologous promoter (that of the mouse p12 gene), e ven in pituitary cells, Recently, we identified in the 5' untranslated region of the gene encoding human cellular retinol binding protein 1 (hCRBP1) a negative regulatory element (Fp1) that also bears an NF1 bi nding site very similar to that of rGH silencer-1, However, although d eletion of Fp1 in the hCRBP1 gene yielded increased CAT activity, poin ting toward a negative regulatory function exerted by this element, it s insertion upstream of the p12 basal promoter results in an impressiv e positive stimulation of CAT gene expression, By exploiting NaDodSO(4 ) gel protein fractionation and renaturation, we identified a 40-kD nu clear protein (designated Bp1) present in GH4C1 cells that binds very strongly to rGH silencer-1 but only weakly to hCRBP1 Fp1, Similarly, w e also detected a 29-kD nuclear factor (designated Bp2) that recognize s exclusively the Fp1 element as its target site, therefore suggesting that different, but likely related, proteins bind these homologous el ements to either activate or repress gene transcription, Although they bind DNA through the recognition of the NF1-like target sequence cont ained on these elements, competition and supershift experiments in ele ctrophoretic mobility shift assays provided evidence that neither of t hese proteins belong to the NF1 family.