Crystal structure of the nosiheptide-resistance methyltransferase of Streptomyces actuosus.

Nosiheptide-resistance methyltransferase (NHR) of Streptomyces actuosus is a class IV methyltransferase of the SpoU family and methylates 23S rRNA at nucleotide adenosine corresponding to A1067 in Escherichia coli. Such methylation is essential for resistance against nosiheptide, a sulfur peptide antibiotic, which is produced by the nosiheptide-producing strain, S. actuosus. Here, we report the crystal structures of NHR and NHR in complex with SAM (S-adenosyl-l-methionine) at 2.0 and 2.1 A resolution, respectively. NHR forms a functional homodimer, and dimerization is required for methyltransferase activity. The monomeric NHR is comprised of the N-terminal RNA binding domain (NTD) and the C-terminal catalytic domain (CTD). Overall, the structure of NHR suggests that the methyltransferase activity is achieved by "reading" the RNA substrate with NTD and "adding" methyl group using CTD. Comprehensive mutagenesis and methyltransferase activity assays reveal critical regions for SAM binding in CTD and loops (L1 and L3) essential for RNA recognition in NTD. Finally, the catalytic mechanism and structural model that NHR recognizes 23S rRNA is proposed based on the structural and biochemical analyses. Thus, our systematic structural studies reveal the substrate recognition and modification by the nosiheptide-resistance methyltransferase.