RNA repair

Genetic bypass of essential RNA repair enzymes in budding yeast


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.