Comparative analysis reveals genomic features of stress-induced transcriptional readthrough

Transcription is a highly regulated process, and stress-induced changes in gene transcription have been shown to play a major role in responses and adaptation to stress. Numerous emerging genome-wide studies reveal prevalent transcription beyond known protein-coding gene loci, generating a variety of new classes of RNAs, most of unknown function. One such class, termed downstream of gene (DoG)-containing transcripts, was reported to result from transcriptional readthrough upon osmotic stress in human cell lines. However, how widespread the readthrough phenomenon is, and what its causes and consequences are, remain elusive. Here we present a systematic genome-wide mapping of transcriptional readthrough, using deep nuclear RNA-seq, comparing heat shock, osmotic and oxidative stress in NIH3T3 mouse fibroblast cells. We observe massive induction of transcriptional readthrough under all stress conditions, with significant, yet not complete overlap of readthrough-induced loci between different conditions. Importantly, our analyses suggest that stress-induced transcriptional readthrough is not a random failure process, but is rather differentially induced across different conditions. Additionally, analyzing public Pol-II occupancy data further supported our findings of stress-induced readthrough. We explore potential regulators and find a role for HSF1 in the induction of a subset of heat shock-induced readthrough transcripts. Furthermore, we examine genomic features of readthrough transcription, and observe a unique chromatin signature typical of DoG-producing regions, suggesting that readthrough transcription is associated with the maintenance of an open chromatin state. RNA profiles of NIH3T3 (mouse embryonic fibroblasts) cells after three stress treatments and control were generated by deep sequencing, in two replicates using Illumina HiSeq 2000.