Molecular basis of the activation of basal histone H1(0) gene expression.

Histone H1(0) is encoded by a gene that is expressed only in cells committed to differentiation. We have previously cloned the Xenopus laevis H1(0) gene and studied elements involved in the regulation of its expression in transfected Xenopus laevis A6 cells, and in microinjected embryos. In this work, in order to understand the basis of the action of these elements, we used an A6 cell nuclear extract and showed that the H1(0) promoter is able to direct efficient in vitro transcription, which is highly dependent on a functional TATA box. However, in contrast to what we observed in vivo, in transfected A6 cells, the in vitro transcription was independent of major regulatory elements, defined in vivo. We then used this in vitro system to reconstitute H1(0) gene regulation. The creation of a repressive environment by the addition of purified histone H1 to the in vitro transcription system allowed us to obtain transcription dependent on the integrity of the regulatory elements. Investigating the basis of this regulation we found that protein-DNA interaction on the proximal promoter region was dependent on the integrity of proximal elements, and moreover the distal regulatory element, the UCE, was able to modulate this interaction. We conclude that the role of these regulatory elements is to maintain the basal TATA-dependent transcription of H1(0) under repressive condition: i.e., H1-mediated repression of transcription, or chromatin assembly in general.