Liver cell specific gene transcription in vitro: the promoter elements HP1 and TATA box are necessary and sufficient to generate a liver-specific promoter.

The hepatocyte-specific promoter element HP1, which is present in several genes specifically expressed in the liver, is active in an in vitro transcription system. The liver-specificity is retained in the in vitro system, as the activity is found in extracts of rat liver or hepatoma cells but is absent in an L-cell extract. Mutational analysis identifies HP1 as a 13 bp element: Two point mutations abolish the function of HP1. This inactivation is correlated with a reduced binding affinity of the transcription factors recognizing HP1. Two other mutants, which reduce the activity of HP1, bind the transcription factors with an affinity identical to the wildtype sequence. This suggests that the binding of the transcription factors is not sufficient for activation of HP1 dependent transcription. The function of HP1 depends on the presence of a TATA box within a distance of some 70 bp. Various TATA sequences are functional and no stereospecific alignment between HP1 and the TATA box is required.