Crystallographic capture of a radical S-adenosylmethionine enzyme in the act of modifying tRNA

RlmN is a dual-specificity RNA methylase that modifies C2 of adenosine 2503 (A2503) in 23S rRNA and C2 of adenosine 37 (A37) in several Escherichia coli transfer RNAs (tRNAs). A related methylase, Cfr, modifies C8 of A2503 via a similar mechanism, conferring resistance to multiple classes of antibiotics. Here, we report the x-ray structure of a key intermediate in the RlmN reaction, in which a Cys¹¹⁸→Ala variant of the protein is cross-linked to a tRNA^Glu substrate through the terminal methylene carbon of a formerly methylcysteinyl residue and C2 of A37. RlmN contacts the entire length of tRNA^Glu, accessing A37 by using an induced-fit strategy that completely unfolds the tRNA anticodon stem-loop, which is likely critical for recognition of both tRNA and ribosomal RNA substrates.