Sequence-Specific Antirepression of Histone H1-Mediated Inhibition of Basal RNA Polymerase II Transcription

Abstract

To understand the principles of control and selectivity in gene expression, the biochemical mechanisms by which promoter- and enhancer-binding factors regulate transcription by RNA polymerase II were analyzed. A general observed repressor of transcription was purified and identified as histone H1. Since many aspects of H1 binding to naked DNA resemble its interaction with chromatin, purified H1 bound to naked DNA was used as a model for the repressed state of the DNA template. Three sequence-specific transcription factors, Sp1, GAL4-VP16, and GAGA factor, were shown to counteract H1-mediated repression (antirepression). In addition, Sp1 and GAL4-VP16, but not the GAGA factor, activated transcription in the absence of H1. Therefore, true activation and antirepression appear to be distinct activities of sequence-specific factors. Furthermore, transcription antirepression by GAL4-VP16 was sustained for several rounds of transcription. These findings, together with previous studies on H1, suggest that H1 participates in repression of the genome in the ground state and that sequence-specific transcription factors induce selected genes by a combination of true activation and release of basal repression that is mediated at least in part by H1.

The BTB domain, found primarily in zinc finger proteins, defines an evolutionarily conserved family that includes several developmentally regulated genes in Drosophila.

“…E(var)3-93D codes for chromatinassociated proteins thought to contribute to the establishment or maintainance of an open, and thus transcriptionally accessible, chromatin conformation (20). The GAGA factor acts as an antirepressor of histone Hi-mediated repression of transcription from the Krdppel promoter and may also be required for an open chromatin conformation (26). It also maps to the same position as gene 62, an enhancer of position-effect variegation (20).…”

A New Class of Histone H2A Mutations in Saccharomyces cerevisiae Causes Specific Transcriptional Defects In Vivo

“…A growing body of evidence from both in vivo and in vitro studies has shown that the structure of chromatin influences gene expression (see references 25 and 53 for reviews). Biochemical experiments have shown that histones can repress transcription in vitro (see reference 53 for a review) and that transcriptional activators can overcome this repression (21,41,45,79,81,82). In vivo experiments in Saccharomyces cerevisiae have shown that the loss of transcriptional activators or of activator binding sites can be suppressed by mutations in histone genes (27,30,37,57).…”

Biochemical analyses of transcriptional regulatory mechanisms in a chromatin context

“…Essentially the same procedure can be used to purify core histones for culture mammalian cells if so desired. Recombinant core histones from various species can be expressed in E. coli and purified as described by Dyer et al (7) Histone H1 ( Figure 1) can be isolated and purified from Drosophila embryos as described by Croston et al (8). The advantage of using Drosophila H1 is the presence of only a single histone H1 isoform.…”