Insights into the catalytic mechanism of 16S rRNA methyltransferase RsmE (m³U1498) from crystal and solution structures.

RsmE is the founding member of a new RNA methyltransferase (MTase) family responsible for methylation of U1498 in 16S ribosomal RNA in Escherichia coli. It is well conserved across bacteria and plants and may play an important role in ribosomal intersubunit communication. The crystal structure in monomer showed that it consists of two distinct but structurally related domains: the PUA (pseudouridine synthases and archaeosine-specific transglycosylases)-like RNA recognition and binding domain and the conserved MTase domain with a deep trefoil knot. Analysis of small-angle X-ray scattering data revealed that RsmE forms a flexible dimeric conformation that may be essential for substrate binding. The S-adenosyl-l-methionine (AdoMet)-binding characteristic determined by isothermal titration calorimetry suggested that there is only one AdoMet molecule bound in the subunit of the homodimer. In vitro methylation assay of the mutants based on the RsmE-AdoMet-uridylic acid complex model showed key residues involved in substrate binding and catalysis. Comprehensive comparisons of RsmE with closely related MTases, combined with the biochemical experiments, indicated that the MTase domain of one subunit in dimeric RsmE is responsible for binding of one AdoMet molecule and catalytic process while the PUA-like domain in the other subunit is mainly responsible for recognition of one substrate molecule (the ribosomal RNA fragment and ribosomal protein complex). The methylation process is required by collaboration of both subunits, and dimerization is functionally critical for catalysis. In general, our study provides new information on the structure-function relationship of RsmE and thereby suggests a novel catalytic mechanism.