Genome-wide mapping of topoisomerase 1 reveals a biphasic role in gene transcription

Tuesday, October 25, 2011 — Poster Session II
Noon – 2:00 p.m.	Natcher Conference Center	NCI	EPIGEN/TRANS/CHROM-2

* FARE Award Winner

Authors

• L Baranello
• D Wojtowicz
• K Cui
• F Kouzine
• T Przytycka
• Y Pommier
• K Zhao
• D Levens

Abstract

Topoisomerase 1 (Top1) regulates DNA topology and enables essential processes such as replication and transcription by transient cleavage of DNA. To determine its precise function, Top1 ChIP-Seq was compared with a new genome wide method to map cleavage sites due to enzymatically active Top1. We found Top1 to localize mainly near the 5’ and 3’ ends of genes and its activity strictly correlates with RNAPII localization, suggesting an association between Top1 and the transcription machinery. Intriguingly, two separate populations of Top1 are found at gene level. The first is distributed along the genes bodies, involved in transcription elongation; the second is localized near the transcription start-sites. Upon Top1 knockdown, only the activity at promoters is strongly impaired simultaneously with accumulation of RNAPII at transcription start-sites. By using campthotecin as in vivo modulator of Top1 processivity, we observed RNAPII to accumulate downstream of the promoters. This indicates that high torsional stress impedes the movement of RNAPII in the gene`s body and affects the reloading of promoters. We show for the first time a biphasic role for Top1 in transcription not only as an elongation factor but as a factor recruited to transcription start-sites.

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