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Transcription by yeast RNA polymerase III and Chromatin Remodeling

par Webmaster - 26 septembre 2007

Orateur :

Purnima Bhargava, Center for Cellular and Molecular Biology, Hyderabad, India.

Salle :


Sujet :

In the budding yeast, 300 genes transcribed by RNA Polymerase III (pol III) give the short non-coding RNAs like tRNA, 5S rRNA etc. The basal transcription factor of pol III, TFIIIC, binds to the two intragenic promoter elements, boxes A and B and recruits the initiation factor TFIIIB 30 bp upstream of the transcription start site, which recruits pol III in turn. U6 snRNA is conserved from yeast to mammals and is transcribed by RNA Polymerase III. Unlike other pol III transcribed genes, the box B in SNR6 (U6 snRNA) gene of S. cerevisiae is found 120 bases downstream of the terminator, positioning it 200 bp away from the box A, in contrast to the optimal 50-60 bp separation of the two found in most tDNAs. The TATA box at –30 bp position can drive the transcription with TFIIIB in vitro on naked DNA templates but box B and TFIIIC are required to relieve the chromatin-mediated repression of SNR6, indicating TFIIIC helps alter the chromatin structure in favor of TFIIIB binding.

Active participation of chromatin in the transcription by pol II is well documented. In contrast, little is known about the role of chromatin in regulation of pol III-transcribed genes. This may be partly because of the prevailing notion that pol III-transcribed genes are devoid of nucleosomes. Recently, several genome-wide studies have reported association of chromatin modifying complexes with pol III-transcribed genes, suggesting that various chromatin remodeling and modification machineries may be required even for the pol III transcription in vivo. Transcription by pol III is tightly regulated in response to growth stage, growth conditions and stress by a central regulator Maf1. Maf1 localizes to yeast SNR6 in vivo and represses SNR6 transcription in vitro. There are no studies available on the response of chromatin structure of pol III-transcribed genes to repression.

Our previous in vitro studies had shown that binding of TFIIIC and TFIIIB to SNR6 leads to the positioning of a nuclosome between A and B boxes as well as upstream of TATA box. We have now strong evidences for the presence of a nucleosome between A and B boxes in vivo. The chromatin structure in vivo shows direct response to repression through a single nucleosome out of the array of nucleosomes in upstream region of the gene. This nucleosme is marked by the histone variant H2A.Z and acetylation of H4K16. We have also identified RSC as the remodeler, which keeps the TATA box nucleosome-free. Our results suggest that at least some of the mechanisms are common for both pol III and pol II to tackle the chromatin barriers for transcription.

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