Tissue-specific regulation of gene expression by the transcription factors Ying-Yang 1 and nuclear factor 1.

In studies on the proximal human p53 promoter, this work has identified a novel transcription factor binding element. It lies about 100 bp upstream of the major reported start site for human p53 gene transcription. DNase I footprinting studies, electromobility shift assay (EMSA) patterns, the effects of mutations, the binding pattern of purified transcription factors and the effects of specific antibodies reveal it to be a composite element which can bind to two transcription factors, YY1 and NF1. YY1 gets its name Ying-Yang 1 due to its ability to both activate and repress transcription depending on the promoter context. NG1, Nuclear Factor 1, comprises a large family of transcription factors, all of which bind to the same consensus sequence. Methylation interference analysis showed the YY1 and NF1 binding elements to be overlapping and therefore the two proteins bind to this element in an independent and mutually exclusive manner. The site is conserved in the human, rat and mouse p53 promoters.

Transient transfection studies of p53 promoter- and mutated p53 promoter-CAT constructs in HeLa cells (which contain both factors) and the overexpression of YY1 or NF1 showed that both YY1 and NF1 activate the p53 promoter and that this composite element contributes to 64% of the basal p53 promoter activity. In order to determine if this composite element is involved in enhanced p53 promoter activity, the promoter was induced by the adenovirus protein, E1A. Over half of the E1A enhancement of the p53 promoter is mediated by YY1 binding to the composite element. Overexpressed YY1 was shown to be able to synergistically activate the p53 promoter with E1A when not specifically bound to DNA.

The occupancy of the site varies in a tissue-specific manner: it binds principally YY1 in nuclear extracts of rat testis and spleen and NF1 in extracts of liver and prostate. This may be a mechanism by which p53 expression can be induced to different levels and rates, depending on the presence and activity of either YY1 or NF1.