Growth hormone receptor (GHR) mRNA variants that differ in the 5 -untranslated regions (5 -UTR) have been isolated in various species. These 5 -UTR variants are generated from the use of alternative promoters and/or alternative splicing. The 5 -UTR 1B is one of the GHR 5 -UTR variants isolated in the bovine but its homologues are also present in other species. The 5 -UTR 1B is a predominant GHR 5 -UTR expressed in many tissues. In the present study, we screened a bovine genomic library and isolated a 1·7 kb bovine GHR genomic sequence including exon 1B and its 5 flanking region from which the GHR 5 -UTR 1B is generated. Using primer extension, two major transcription start sites were mapped in the bovine exon 1B. Transient transfection analysis of the 5 flanking region of exon 1B confirmed its promoter activity (termed P2) in both Hep G2 and BHK-21 cells. Furthermore, analysis of deletion promoterreporter constructs found that the basal activity of P2 resided in the proximal region of P2. DNase I footprinting analysis and electromobility shift assay (EMSA) identified the ubiquitous transcription factor Sp1 as the binding protein to a GC box-containing DNA element within the proximal P2. Deletion of the GC box greatly reduced the activity of P2 in cell lines. The GC box-containing site also appeared to bind Sp1 in the nuclear extracts from diverse bovine tissues. This suggests that interactions of Sp1 with the GC box-containing element in the proximal region of P2 may be part of the mechanism for the expression of the bovine GHR 5 -UTR 1B in diverse tissues.
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