UC San Diego

Eukaryotic RNA processing and quality control involves factors that add 3' end modifications on RNA transcripts. However, the importance of most modifications of small non-coding RNAs (sncRNAs) is poorly understood. The phosphodiesterase Usb1 and the non-canonical poly(A) polymerase TENT2 are known to modify the 3’ ends of RNA polymerase-III (pol-III) transcripts U6 and 7SL, respectively. However, Usb1 and TENT2’s targets have not been systematically identified. To better understand the scope of 3' end modifications of sncRNAs, I analyzed differences in post-transcriptional 3' end modifications between pol-III and pol-II sncRNAs and found that adenylation can be observed at the mature 3’ end of most pol-III transcripts at a level significantly higher than for pol-II transcripts. For all pol-III transcripts, only a subset of the population is adenylated at steady state, and adenylation is generally restricted to a single adenosine. By quantifying differences in 3' end adenylation via 3' end sequencing in the presence or absence of TENT2, I identified several pol-III transcripts as targets of TENT2, including 7SL, U6atac, and Y RNAs. As Usb1 is known to add 3’ end cyclic phosphates to U6 snRNA, we hypothesized that Usb1 may be responsible for modifying additional pol-III transcripts. However, my experiments did not show evidence for cyclic phosphates on additional tested pol-III transcripts. My results demonstrate that adenylation can be observed for most pol-III sncRNAs and that TENT2 is responsible for the adenylation of a subset of these RNAs, suggesting a general role for 3’ monoadenylation in pol-III transcript processing or function.