Fractionation of the general RNA polymerase II transcription factors from Drosophila embryos

TFIID can be rate limiting in vivo for TATA-containing, but not TATA-lacking, RNA polymerase II promoters

We have studied the effect of exogenous expression of the basal transcription factor TFIID on the activities of several different TATA-containing and TATA-lacking promoters. Overexpression of TFIID from a transfected plasmid in Drosophila Schneider cells resulted in substantial concentration-dependent increases in expression from a cotransfected minimal TATA-containing promoter. Overexpression of TFIID activated expression from all TATA-containing promoters tested, with the maximum level of activation being inversely proportional to the strength of the promoter. In contrast, expression from TATA-less promoters was not enhanced, and could in fact be reduced, by increased expression of TFIID. Consistent with these findings overexpression of TFIID had opposite effects on Sp1-mediated activation observed from minimal synthetic promoters consisting of Sp1-binding sites and either a TATA box or initiator element. We discuss the significance of these results in terms of the role of TFIID in the initiation of transcription and as a possible regulatory target for expression from TATA-containing promoters, as well as the role TFIID may play in expression from TATA-less promoters.

Role of nucleosomal cores and histone H1 in regulation of transcription by RNA polymerase II

The relation between chromatin structure and transcriptional activity was examined by in vitro transcription analysis of chromatin reconstituted in the absence or presence of histone H1. To maintain well-defined template DNA, purified components were used in the reconstitution of chromatin. Reconstitution of nucleosomal cores to an average density of 1 nucleosome per 200 base pairs of DNA resulted in a mild reduction of basal RNA polymerase II transcription to 25 to 50 percent of that obtained with naked DNA templates. This nucleosome-mediated repression was due to nucleosomal cores located at the RNA start site and could not be counteracted by the sequence-specific transcription activators Sp1 and GAL4-VP16. When H1 was incorporated into the chromatin at 0.5 to 1.0 molecule per nucleosome (200 base pairs of DNA), RNA synthesis was reduced to 1 to 4 percent of that observed with chromatin containing only nucleosomal cores, and this H1-mediated repression could be counteracted by the addition of Sp1 or GAL4-VP16 (antirepression). With naked DNA templates, transcription was increased by a factor of 3 and 8 by Sp1 and GAL4-VP-16, respectively (true activation). With H1-repressed chromatin templates, however, the magnitude of transcriptional activation mediated by Sp1 and GAL4-VP16 was 90 and more than 200 times higher, respectively, because of the combined effects of true activation and antirepression. The data provide direct biochemical evidence that support and clarify previously proposed models in which there is depletion or reconfiguration of nucleosomal cores and histone H1 at the promoter regions of active genes.

Isolation of coactivators associated with the TATA-binding protein that mediate transcriptional activation

A key step in the regulation of transcription involves interactions between promoter-selective factors and various components of the transcriptional apparatus. Here we report the requirements for transcriptional activation directed by NTF-1, a developmentally regulated transcription factor in Drosophila. Reconstituted transcription with fractionated Drosophila basal factors reveals that activation by NTF-1 requires factors present in the endogenous TFIID fraction that are distinct from the purified TATA-binding protein (TBP). Glycerol gradient sedimentation and immunoprecipitation analyses indicate that TFIID is a multiprotein complex containing TBP and at least six tightly bound TBP-associated factors (TAFs). Preparations of TBP lacking TAFs after fractionation with denaturants no longer support activation by NTF-1 but retain basal level activity. Addition of immunopurified and renatured TAFs to free TBP restores the ability of NTF-1 to activate transcription without influencing basal transcription. These results suggest that one or more of the TAF polypeptides confer coactivator function.

A general method for purification of H1 histones that are active for repression of basal RNA polymerase II transcription

H1 histones were purified from extracts of salt-treated nuclei as a co-product of RNA polymerase II transcription factors from both Drosophila embryos and HeLa cells by a simple and general method. This procedure was also used to purify H1 as co-product of the core histones from calf thymus. The key steps in this purification exploit the solubility of H1 in 2.26 M ammonium sulfate and the chromatographic properties of the highly charged H1 molecules on a phenyl-Sepharose resin. H1 that is prepared by this procedure is active for in vitro repression of basal RNA polymerase II transcription. This method provides a new means of purifying H1 by a mild procedure that is likely to be generally useful for studies of transcription and chromatin structure.