Intergenic splicing-stimulated transcriptional readthrough is suppressed by nonsense-mediated mRNA decay in Arabidopsis

Recent emerging evidence has shown that readthrough transcripts (RTs), including polycistronic mRNAs, are also transcribed in eukaryotes. However, the post-transcriptional regulation for these remains to be elucidated. Here, we identify 271 polycistronic RT-producing loci in Arabidopsis. Increased accumulation of RTs is detected in the nonsense-mediated mRNA decay (NMD)-deficient mutants compared with wild type, and the second open reading frames (ORFs) of bicistronic mRNAs are rarely translated in contrast to the first ORFs. Intergenic splicing (IS) events which occur between first and second genes are seen in 158 RTs. Splicing inhibition assays suggest that IS eliminates the chance of transcription termination at the polyadenylation sites of the first gene and promotes accumulation of RTs. These results indicate that RTs arise from genes whose transcription termination is relatively weak or attenuated by IS, but NMD selectively degrades them. Ultimately, this report presents a eukaryotic strategy for RNA metabolism.

Automated summary

Recent evidence suggests the existence of readthrough transcripts (RTs), including polycistronic mRNAs, in eukaryotes. This study identifies 271 loci producing polycistronic RTs in Arabidopsis. The accumulation of RTs is increased in NMD-deficient mutants compared to the wild type. Additionally, the translation of the second open reading frames (ORFs) of bicistronic mRNAs is rare. Intergenic splicing (IS) events occur in 158 RTs, and inhibiting splicing promotes the accumulation of RTs.