Previous studies on the N-terminal domain of Lon proteases have not clearly identified its function. Here we constructed randomly chosen N-terminal-truncated mutants of the Lon protease from Brevibacillus thermoruber WR-249 to elucidate the structure-function relationship of this domain. Mutants lacking amino acids from 1 to 247 of N terminus retained significant peptidase and ATPase activities, but lost approximately 90% of protease activity. Further truncation of the protein resulted in the loss of all three activities. Mutants lacking amino acids 246-259 or 248-256 also lost all activities and quaternary structure. Our results indicated that amino acids 248-256 (SEVDELRAQ) are important for the full function of the Lon protease.