In the unfolded protein response (UPR), protein-folding stress in the endoplasmic reticulum (ER) activates a large transcriptional program to increase ER folding capacity. During the budding yeast UPR, the trans-ER-membrane kinase-endoribonuclease Ire1 excises an intron from the HAC1 mRNA and the exon cleavage products are ligated and translated to a transcription factor that induces hundreds of stress-response genes. HAC1 cleavage by Ire1 is thought to be the rate limiting step of its processing. Using cells with mutations in RNA repair and decay enzymes, we show that phosphorylation of two different HAC1 splicing intermediates by Trl1 RNA 5′-kinase is required for their degradation by the 5′→3′ exonuclease Xrn1 to enact opposing effects on the UPR. Kinase-mediated decay (KMD) of cleaved HAC1 3′-exon competes with its ligation to limit productive splicing and suppress the UPR, whereas KMD of the excised intron activates HAC1 translation, likely by relieving an inhibitory base-pairing interaction between the intron and 5′-untranslated region. We also found that ligated but 2′-phosphorylated HAC1 mRNA is endonucleolytically cleaved, yielding a KMD intermediate with both 5′- and 2′-phosphates at its 5′-end that inhibit 5′→3′ decay, and suggesting that Ire1 initiates the degradation of incompletely processed HAC1s to proofread ligation or attenuate the UPR. These multiple decay events expand the scope of RNA-based regulation in the budding yeast UPR and may have implications for the control of the metazoan UPR by mRNA processing.

Single-cell RNA sequencing (scRNA-seq) methods generate sparse gene expression profiles for thousands of single cells in a single experiment. The information in these profiles is sufficient to classify cell types by distinct expression patterns but the high complexity of scRNA-seq libraries often prevents full characterization of transcriptomes from individual cells. To extract more focused gene expression information from scRNA-seq libraries, we developed a strategy to physically recover the DNA molecules comprising transcriptome subsets, enabling deeper interrogation of the isolated molecules by another round of DNA sequencing. We applied the method in cell-centric and gene-centric modes to isolate cDNA fragments from scRNA-seq libraries. First, we resampled the transcriptomes of rare, single megakaryocytes from a complex mixture of lymphocytes and analyzed them in a second round of DNA sequencing, yielding up to 20-fold greater sequencing depth per cell and increasing the number of genes detected per cell from a median of 1,313 to 2,002. We similarly isolated mRNAs from targeted T cells to improve the reconstruction of their VDJ-rearranged immune receptor mRNAs. Second, we isolated CD3D mRNA fragments expressed across cells in a scRNA-seq library prepared from a clonal T cell line, increasing the number of cells with detected CD3D expression from 59.7% to 100%. Transcriptome resampling is a general approach to recover targeted gene expression information from single-cell RNA sequencing libraries that enhances the utility of these costly experiments, and may be applicable to the targeted recovery of molecules from other single-cell assays.

RNA repair enzymes catalyze rejoining of an RNA molecule after cleavage of phosphodiester linkages. RNA repair in budding yeast is catalyzed by two separate enzymes that process tRNA exons during their splicing and HAC1 mRNA exons during activation of the unfolded protein response. The RNA ligase Trl1 joins 2′,3′-cyclic phosphate and 5′-hydroxyl RNA fragments, creating a new phosphodiester linkage with a 2′-phosphate at the junction. The 2′-phosphate is subsequently removed by the 2′-phosphotransferase Tpt1, which catalyzes phosphate transfer to NAD+, producing nicotinamide and a unique ADP ribose metabolite. We bypassed the essential functions of TRL1 and TPT1 in budding yeast by expressing “pre-spliced,” intronless versions of the ten normally intron-containing tRNAs, indicating this repair pathway does not have additional essential functions. Consistent with previous studies, expression of intronless tRNAs failed to rescue the growth of cells with deletions in components of the SEN complex, implying an additional essential role for the splicing endonuclease. The trl1∆ and tpt1∆ mutants accumulate tRNA and HAC1 splicing intermediates indicative of specific RNA repair defects and are hypersensitive to drugs that inhibit translation. As expected, failure to induce the unfolded protein response in trl1∆ cells grown with tunicamycin is lethal owing to their inability to ligate HAC1 after its cleavage by Ire1. In contrast, tpt1∆ mutants grow in the presence of tunicamycin despite reduced accumulation of spliced HAC1, suggesting that ligated but 2′-phosphorylated mRNA is decoded by the ribosome. Finally, we optimized a PCR-based method to detect RNA 2′-phosphate modifications and show that they are present on ligated HAC1 mRNA. These RNA repair mutants enable new studies of the role of RNA repair in cellular physiology.

New tools for reproducible exploratory data analysis of large datasets are important to address the rising size and complexity of genomic data. We developed the valr R package to enable flexible and efficient genomic interval analysis. valr leverages new tools available in the ‘tidyverse’, including dplyr. Benchmarks of valr show it performs similar to BEDtools and can be used for interactive analyses and incorporated into existing analysis pipelines.

RNA cleavage by some endoribonucleases and self-cleaving ribozymes produces RNA fragments with 5’-hydroxyl (5’-OH) and 2’,3’-cyclic phosphate termini. To identify 5’-OH RNA fragments produced by these cleavage events, we exploited the unique ligation mechanism of Escherichia coli RtcB RNA ligase to attach an oligonucleotide linker to RNAs with 5’-OH termini, followed by steps for library construction and analysis by massively parallel DNA sequencing. We applied the method to RNA from budding yeast and captured known 5’-OH fragments produced by tRNA Splicing Endonuclease (SEN) during processing of intron-containing pre-tRNAs and by Ire1 cleavage of HAC1 mRNA following induction of the unfolded protein response (UPR). We identified numerous novel 5’-OH fragments derived from mRNAs: some 5’-OH mRNA fragments were derived from single, localized cleavages, while others were likely produced by multiple, distributed cleavages. Many 5’-OH fragments derived from mRNAs were produced upstream of codons for highly electrostatic peptides, suggesting that the fragments may be generated by co-translational mRNA decay. Several 5’-OH RNA fragments accumulated during the induction of the UPR, some of which share a common sequence motif that may direct cleavage of these mRNAs. This method enables specific capture of 5’-OH termini and complements existing methods for identifying RNAs with 2’,3’-cyclic phosphate termini.