Identification of a selective DNA ligase for accurate recognition and ultrasensitive quantification of N6-methyladenosine in RNA at one-nucleotide resolution.

N6-Methyladenosine (m6A) is the most frequent post-transcriptional modification in RNA, and it plays a critical role in biological processes. The functions of m6A remain largely unexplored due to a lack of highly sensitive methods to quantitatively determine the m6A modification fraction at a precise location. Here, we first reveal that T3 DNA ligase has significant selectivity towards the m6A modification. On the basis of the new finding, we establish an ultrasensitive quantitation assay for accurately determining m6A at one-nucleotide resolution in RNA. With the proposed assay, as low as 4 fM RNA containing m6A can be determined and the selectivity is up to 54.1-fold to discriminate m6A against unmodified adenosine (A). The sensitivity has been improved about 106-fold so the proposed method can be successfully employed to accurately determine m6A in real biological samples, even in low abundance RNA.