RrmJ (FtsJ) is the first 23S rRNA methyltransferase under heat shock control. It is highly conserved in prokaryotes, archaea and eukaryotes. In prokaryotes it is responsible for the 2′-O-methylation of uridine 2552 in the A-loop of the 23S rRNA, late in the maturation process of the ribosome. This methylation seems to be very important for the stability of the ribosome. In the absence of functional RrmJ, ribosomes prematurely dissociate in their 50S and 30S subunits. 1.5 Å structure of the methyltransferase RrmJ in complex with its methyl cofactor S-Adenosyl methionine (AdoMet). Diagram of RrmJ’s tertiary fold highlighting secondary structure elements. b-strands are shown in yellow, a-helices are shown in cyan and the AdoMet is shown in ball and stick representation.