Abstract
BackgroundRNA polymerase (pol) III transcribes a unique class of genes with intra-genic promoters and high transcriptional activity. The major contributors to the pol III transcriptome, tRNAs genes are found scattered on all chromosomes of yeast. A prototype tDNA of <150 bp length, is generally considered nucleosome-free while some pol III-transcribed genes have been shown to have nucleosome-positioning properties.ResultsUsing high resolution ChIP-chip and ChIP-seq methods, we found several unique features associated with nucleosome profiles on all tRNA genes of budding yeast, not seen on nucleosome-dense counterparts in fission yeast and resting human CD4+ T cells. The nucleosome-free region (NFR) on all but three yeast tDNAs is found bordered by an upstream (US) nucleosome strongly positioned at −140 bp position and a downstream (DS) nucleosome at variable positions with respect to the gene terminator. Perturbation in this nucleosomal arrangement interferes with the tRNA production. Three different chromatin remodelers generate and maintain the NFR by targeting different gene regions. Isw1 localizes to the gene body and makes it nucleosome-depleted, Isw2 maintains periodicity in the upstream nucleosomal array, while RSC targets the downstream nucleosome. Direct communication of pol III with RSC serves as a stress-sensory mechanism for these genes. In its absence, the downstream nucleosome moves towards the gene terminator. Levels of tRNAs from different families are found to vary considerably as different pol III levels are seen even on isogenes within a family. Pol III levels show negative correlation with the nucleosome occupancies on different genes.ConclusionsBudding yeast tRNA genes maintain an open chromatin structure, which is not due to sequence-directed nucleosome positioning or high transcription activity of genes. Unlike 5′ NFR on pol II-transcribed genes, the tDNA NFR, which facilitates tDNA transcription, results from action of chromatin remodeler Isw1, aided by Isw2 and RSC. The RSC-regulated nucleosome dynamics at the 3′ gene-end serves as a novel regulatory mechanism for pol III transcription in vivo, probably by controlling terminator-dependent facilitated recycling of pol III. Salient features of yeast tDNA chromatin structure reported in this study can explain the basis of the novel non-transcriptional roles ascribed to tDNAs.
Highlights
RNA polymerase III transcribes a unique class of genes with intra-genic promoters and high transcriptional activity
We found that majority of Isw2-occupied tDNAs are enriched for Isw1 (Additional file 2: Figure S2C) but on different gene regions (Figure 2D)
The study enhances our perception about how nucleosome dynamics participates in pol III transcription regulation and how their arrangements on different gene classes have evolved to match the requirements. It is fully evident from this data that a strongly positioned upstream (US) nucleosome, the nucleosome-free region (NFR) on tDNAs and nucleosome downstream of the gene are not sequence-directed, rather the result of a fine balance between specific activities of the chromatin remodelers RSC and ISWI
Summary
RNA polymerase (pol) III transcribes a unique class of genes with intra-genic promoters and high transcriptional activity. RNA polymerase (pol) III transcribes a heterogeneous set of small non-coding RNA genes with majority constituted of tRNA genes (70–130 bp). Pol III-transcribed genes are found in comparatively nucleosome-depleted intergenic regions [4,6], which gives the general impression that chromatin may not have much to do with their transcription. Activation of pol III transcription from several sites after global nucleosome depletion in yeast cells, suggests otherwise [12]. Presence of chromatin remodelers RSC and Isw on pol III-transcribed genes has been reported [13,14]. Recent studies with human pol III-transcribed genes have reported that the presence of pol II and pol II-specific chromatin marks near pol III-transcribed genes generates an active chromatin milieu for them [18,19,20]
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