Independent recruitment of mediator and SAGA by the activator Met4

Mediator is a key RNA polymerase II (Pol II) cofactor in the regulation of eukaryotic gene expression. It is believed to function as a coactivator linking gene-specific activators to the basal Pol II initiation machinery. In support of this model, we provide evidence that Mediator serves in vivo as a coactivator for the yeast activator Met4, which controls the gene network responsible for the biosynthesis of sulfur-containing amino acids and S-adenosylmethionine. In addition, we show that SAGA (Spt-Ada-Gcn5-acetyltransferase) is also recruited to Met4 target promoters, where it participates in the recruitment of Pol II by a mechanism involving histone acetylation. Interestingly, we find that SAGA is not required for Mediator recruitment by Met4 and vice versa. Our results provide a novel example of functional interplay between Mediator and coactivators involved in histone modification.     

Tissue- and gene-specific recruitment of steroid receptor coactivator-3 by thyroid hormone receptor during development

Numerous coactivators that bind nuclear hormone receptors have been isolated and characterized in vitro. Relatively few studies have addressed the developmental roles of these cofactors in vivo. By using the total dependence of amphibian metamorphosis on thyroid hormone (T3) as a model, we have investigated the role of steroid receptor coactivator 3 (SRC3) in gene activation by thyroid hormone receptor (TR) in vivo. First, expression analysis showed that SRC3 was expressed in all tadpole organs analyzed. In addition, during natural as well as T3-induced metamorphosis, SRC3 was up-regulated in both the tail and intestine, two organs that undergo extensive transformations during metamorphosis and the focus of the current study. We then performed chromatin immunoprecipitation assays to investigate whether SRC3 is recruited to endogenous T3 target genes in vivo in developing tadpoles. Surprisingly, we found that SRC3 was recruited in a gene- and tissue-dependent manner to target genes by TR, both upon T3 treatment of premetamorphic tadpoles and during natural metamorphosis. In particular, in the tail, SRC3 was not recruited in a T3-dependent manner to the target TRbetaA promoter, suggesting either no recruitment or constitutive association. Finally, by using transgenic tadpoles expressing a dominant negative SRC3 (F-dnSRC3), we demonstrated that F-dnSRC3 was recruited in a T3-dependent manner in both the intestine and tail, blocking the recruitment of endogenous coactivators and histone acetylation. These results suggest that SRC3 is utilized in a gene- and tissue-specific manner by TR during development.