Structure of the Thermus thermophilus 16 S rRNA methyltransferase KsgA

Post‐transcriptional modification of ribosomal RNA occurs in all kingdoms of life. The S ‐adenosyl‐L‐methionine‐dependent methyltransferase KsgA introduces the most highly conserved ribosomal RNA modification, the dimethylation of A1518 and A1519 of 16S rRNA. Loss of this dimethylation confers resistance to the antibiotic kasugamycin. Here, we report biochemical studies and high‐resolution crystal structures of KsgA from Thermus thermophilus in complex with 5′‐methylthioadenosine . A structural comparison with related methyltransferases reveals the presence of a novel N‐terminal extension. Several mobile structure elements are observed that restrict access to the active site. A comparison with other rRNA modifying enzymes shows significantly different modes for substrate recognition.