Substrate analysis showed that RlmE (formerly FtsJ, RrmJ) methylates the 2′-O ribose of the universally conserved U2552 in the so-called A-loop (hairpin 92) of the Peptidyl Transferase Center (domain V) of 23S rRNA. E. coli RlmE is active only on the ribosome and the 50 S ribosomal subunit, but is inactive on free rRNA. AdoMet is the methyl group donor. The enzyme possess a TRAM domain. The homologous S.cerevisiae mitochondrial enzyme is Mrm2. In cytoplasmic large subunit rRNA, the equivalent Um2921 (2552 in E. coli numbering) is catalyzed by C/D box RNP, using snR52 as guide-RNA.