RESUMEN
Eukaryotic gene expression starts off from a largely obstructive chromatin substrate that has to be rendered accessible by regulated mechanisms of chromatin remodeling. The yeast PHO5 promoter is a well known example for the contribution of positioned nucleosomes to gene repression and for extensive chromatin remodeling in the course of gene induction. Recently, the mechanism of this remodeling process was shown to lead to the disassembly of promoter nucleosomes and the eviction of the constituent histones in trans. This finding called for a histone acceptor in trans and thus made histone chaperones likely to be involved in this process. In this study we have shown that the histone chaperone Asf1 increases the rate of histone eviction at the PHO5 promoter. In the absence of Asf1 histone eviction is delayed, but the final outcome of the chromatin transition is not affected. The same is true for the coregulated PHO8 promoter where induction also leads to histone eviction and where the rate of histone loss is reduced in asf1 strains as well, although less severely. Importantly, the final extent of chromatin remodeling is not affected. We have also presented evidence that Asf1 and the SWI/SNF chromatin remodeling complex work in distinct parallel but functionally overlapping pathways, i.e. they both contribute toward the same outcome without being mutually strictly dependent.
Asunto(s)
Fosfatasa Alcalina/metabolismo , Proteínas de Ciclo Celular/metabolismo , Regulación Fúngica de la Expresión Génica , Histonas/metabolismo , Chaperonas Moleculares/metabolismo , Regiones Promotoras Genéticas , Proteínas de Saccharomyces cerevisiae/metabolismo , Fosfatasa Ácida , Adenosina Trifosfatasas , Fosfatasa Alcalina/genética , Proteínas de Ciclo Celular/genética , Ensamble y Desensamble de Cromatina , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/metabolismo , Chaperonas Moleculares/genética , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Proteínas de Saccharomyces cerevisiae/genética , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Activación TranscripcionalRESUMEN
In yeast, remodeling of PHO5 promoter chromatin upon activation is accompanied by transient hyperacetylation and subsequent eviction of histones from the promoter in trans. In the course of rerepression, nucleosomes have to be reassembled on the promoter. We have analyzed where the histones for reassembly of the inactive promoter chromatin come from. The use of a strain with two differently tagged and differently regulated versions of histone H3 allowed us to discriminate between histones originating from the chromatin fraction and histones arising from the soluble histone pool. In this way, we show that the incorporated histones originate from a source in trans. Promoter closure occurs very rapidly, and the histone chaperones Asf1 and Hir1 as well as the SWI/SNF nucleosome remodeling complex appear to be important for rapid reassembly of nucleosomes at the PHO5 promoter.