Pol II transcription termination in Arabidopsis by the nuclear exoribonuclease XRN3

Two nuclear 5'-3' exonucleases XRN2/3 in Arabidopsis thaliana are homologs of the yeast and human Rat1/Xrn2, which are involved in degradation and processing of several classes of nuclear RNAs and in transcription termination of RNA polymerase II. Here we show that knockdown of XRN3 leads to altered expression of several hundred of the Arabidopsis genes and accumulation of new non-coding RNAs. Using strand-specific short read sequencing we reveal a widespread accumulation of intergenic transcripts in xrn3 mutants. These non-coding XAT (xrn3-associated transcripts) RNAs are generated by Pol II read-through transcription and are usually polyadenylated and lack the 5' cap structure. We show that XRN3-mediated changes in expression of a subset of genes are related to XAT transcription and may be enhanced by XAT-mRNA chimeras produced in xrn3 plants while antisense XATs may trigger siRNA production. Our results highlight the important role of the Rat1/Xrn2 5'-3' exoribonucleases in the torpedo mechanism of Pol II transcription termination and show that a global disturbance in this process significantly impacts both gene expression and transcriptome integrity.

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Krzyszton M, Zakrzewska-Placzek M, Kwasnik A, Dojer N, Karlowski W, Kufel J