Abstract
Pseudourine (Ψ) is the most abundant cellular RNA modification, present in tRNA, rRNA, snRNA, mRNA, long noncoding RNA (lncRNA), and others. Ψ sites and fractions are dynamically regulated in stress response and across development stages. Although high throughput Ψ sequencing methods based on N-Cyclohexyl-N'-(2-morpholinoethyl)carbodiimide (CMC) reaction are available for Ψ detection transcriptome-wide, a simple method for the analysis of specific, targeted Ψ sites and their fraction quantitation is needed to better investigate Ψ function. Here, we describe an RT-PCR and gel electrophoresis based method that can sensitively and quantitatively assess Ψ at single-nucleotide resolution in mRNA/lncRNA, termed CMC-RT and ligation assisted PCR analysis of Ψ modification (CLAP). The principle of the CMC-method is the reverse transcription stop induced by the CMC-Ψ adduct. In CLAP, CMC reaction is first carried out with the RNA sample. Reverse transcription using a non-processive RT produces two cDNA products for each RNA transcript, one with the 3' end at the Ψ site, the other read-through product from the unmodified RNA. Using splint oligonucleotide assisted site-specific ligation, these two cDNA products are then visualized on a gel as two distinct PCR products in the same lane corresponding to the Ψ-modified and unmodified target site. CLAP validates Ψ sites identified by high throughput sequencing, quantifies Ψ levels in mRNA and lncRNA, and enables convenient and rapid investigation on the function and mechanism of the Ψ modification.