Human 5p-tailed Mirtrons are Processed by RNaseP

Approximately a thousand microRNAs (miRNAs) are documented from human cells. A third appear to transit non-canonical pathways that typically bypass processing by Drosha, the dedicated nuclear miRNA producing enzyme. The largest class of non-canonical miRNAs are mirtrons which eschew Drosha to mature through spliceosome activity. While mirtrons are found in several configurations, the vast majority of human mirtron species are 5-tailed. For these mirtrons, a 3 splice site defines the 3 end of their hairpin precursor while a "tail" of variable length separates the 5 base of the hairpin from the nearest splice site. How this tail is removed is not understood. Here we examine sequence motifs in 5-tailed mirtrons and interactions with RNA turnover processes to characterize biogenesis processes. Through studying the high confidence 5-tailed mirtron, hsa-miR-5010, we identify RNaseP as necessary and sufficient for "severing" the 5 tail of this mirtron. Further, depletion of RNaseP activity globally decreased 5-tailed mirtron expression implicating this endoribonuclease in biogenesis of the entire class. Moreover, as 5-tailed mirtron biogenesis appears to be connected to tRNA processing we found a strong correlation between accumulation of tRNA fragments (tRFs) and 5-tailed mirtron abundance. This suggests that dysregulation of tRNA processing seen in cancers may also impact expression of the [~]400 5-tailed mirtrons encoded in the human genome. SUMMARYAbundant non-canonical human miRNAs referred to as tailed mirtrons are processed by RNaseP, which "severs" tail nucleotides to yield a precursor hairpin suitable for Dicer processing. Biogenesis of these miRNAs is correlated with tRFs, which are also products of RNaseP processing.