A Quantitative Sequencing Method for 5-Formylcytosine in RNA

5-Formylcytosine (f⁵C) modification is present in human mitochondrial methionine tRNA (mt-tRNA^Met) and cytosolic leucine tRNA (ct-tRNA^Leu), with their formation mediated by NSUN3 and ALKBH1. f⁵C has also been detected in mRNA of yeast and human cells, but its transcriptome-wide distribution in mammals has not been studied. Here we report f⁵C-seq, a quantitative sequencing method to map f⁵C transcriptome-wide in HeLa and mouse embryonic stem cells (mESCs). We show that f⁵C in RNA can be reduced to dihydrouracil (DHU) by pic-borane, and DHU can be exclusively read as T during reverse transcription (RT) reaction, allowing the detection and quantification of f⁵C sites by a unique C-to-T mutation signature. We validated f⁵C-seq by identifying and quantifying the two known f⁵C sites in tRNA, in which the f⁵C modification fractions dropped significantly in ALKBH1-depleted cells. By applying f⁵C-seq to chromatin-associated RNA (caRNA), we identified several highly modified f⁵C sites in HeLa and mouse embryonic stem cells (mESC).