Characterization of the human N-ras promoter region

Abstract

Overexpression of ras proto-oncogenes has been implicated in cancer development. We therefore initiated a study of the human N-ras promoter to determine the regions that control N-ras expression and their potential for interaction with DNA-binding proteins. N-ras CAT constructs were stably integrated into K562 cells by electric field-mediated gene transfer in order to determine functional regions within the human N-ras promoter. A significant proportion of promoter activity was found to lie within a 439 bp fragment comprising an untranslated exon (exon 1) with the adjacent 5' sequence and a small CpG island. A 109 bp [corrected] fragment at the 5' end of exon 1 was essential for promoter activity, while a 45 bp [corrected] deletion from within this region decreased promoter activity by two-thirds. Unlike the human H-ras and mouse K-ras promoters, the N-ras promoter did not exhibit bidirectional activity. DNAse footprinting of the 439 bp fragment revealed seven protected regions, many of which contain sequences homologous to known DNA-binding protein sites (MLTF/myc, CREB/ATF, AP-1, AP-2, myb and E4TF1). In contrast, four putative Sp1 sites did not footprint. Using purified MLTF and appropriate competitors in gel shift and DNAase footprinting assays, we demonstrated binding of MLTF to the MLTF consensus sequence within exon 1.