Figure 1.

m⁶A disrupts dsRNA duplex and favours formation of a shorter duplex with m⁶A stacked on the 5ʹ end of the dsRNA; m⁶A reduces dsRNA structure and thereby reduces ADAR RNA editing. A. In free nucleotide m⁶A, the methyl group adopts the relaxed syn conformation to avoid a steric clash with the N⁷ hydrogen that occurs in the anti conformation of the methyl group. B. In the m⁶A:U Watson-Crick base pair in dsRNA, the m⁶A methyl group is forced into the unfavourable, high energy, anti conformation having the steric clash. C. If local dsRNA structure is sufficiently weak, part of the dsRNA melts and m⁶A, now at the end of a shorter dsRNA duplex, relaxes to the syn conformation. m⁶A with the methyl in the syn conformation also base stacks on the end of the duplex more favourably than a G would, helping to stabilize the shorter dsRNA. In the presence of the METTL3 writer complex, m⁶A weakens dsRNA duplexes, either making them shorter or disrupting them entirely if the residual paired stretches are too short. In a Mettl3 mutant or knockdown the unmethylated RRACH sequence can become part of a more stable extended dsRNA helix, increasing the efficiency of ADAR editing at sites already edited in METTL3 wildtype; also, the extended dsRNA may make new sites available for ADAR RNA editing. ADARs require at least 17 base pairs of dsRNA for editing activity