Binding of the Sp1 Transcription Factor by the Human Harvey ras 1 Proto-Oncogene Promoter

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Abstract
Members of the ras gene family encode proteins that when overproduced or mutated can transform immortalized mammalian cells. It is therefore important to understand the mechanisms by which the ras genes are regulated. The promoter region of the human Harvey ras proto-oncogene c-Ha- ras 1 initiates RNA transcription at multiple sites and contains repeated copies of the hexanucleotide GGGCGG and its inverted complement CCGCCC, referred to as GC boxes. These GC boxes consist of sequences identical to those found in the SV40 early promoter, where the human cellular transcriptional factor Sp1 binds. Footprinting analysis with deoxyribonuclease I was used to show that Sp1 binds to six GC box sequences within the c-Ha- ras 1 promoter. An in vivo transfection assay showed competition between the 21-base pair repeats of the SV40 promoter and the c-Ha- ras 1 promoter for common regulatory factors. In this system the presence of Sp1 is apparently required for c-Ha- ras 1 transcription. Analysis of deletions of the c-Ha- ras 1 promoter region by means of a transient expression assay revealed that the three Sp1 binding sites closest to the RNA start sites were sufficient for full transcriptional activity.