The histone variant H2A.Z interconverts two stable epigenetic chromatin states

摘要

pThe nucleosomes occupying the chromosomal start sites of transcription contain the histone H2A variant H2A.Z in place of H2A. Upon galactose induction, nucleosomes are evicted from the iGAL1/i locus in iSaccharomyces cerevisiae/i cells. H2A.Z (which is encoded by the iHTZ1/i gene in iS. cerevisiae/i) is required for the eviction of the iGAL1/i promoter nucleosome and for the transcriptional activation of the iGAL1/i gene; however, histones are also important for transcriptional repression and we asked in the present paper if H2A.Z also plays a role in the glucose repression of the iGAL1/i promoter. With the help of a fusion of the iURA3/i ORF (open reading frame) to the iGAL1/i promoter, we were able to detect two different epigenetic transcription states of the iGAL1/i promoter in glucose-grown cells lacking H2A.Z: a repressed state that is occupied by a H2A-containing nucleosome and a derepressed state that is nucleosome-free. These two chromatin states are inherited stably through many cell divisions. According to the model described in the present paper, the role of H2A.Z is to facilitate the addition and removal of promoter nucleosomes and to prevent the formation of unfavourable stable epigenetic chromatin structures, which are not in accordance with the environmental conditions./p