The standalone aminopeptidase PepN catalyzes the maturation of blasticidin S from leucylblasticidin S

The peptidyl nucleoside blasticidin S (BS) isolated from Streptomyces griseochromogenes was the first non-mercurial fungicide used on a large scale to prevent rice blast. In the biosynthesis of BS, leucylblasticidin S (LBS) was suggested as the penultimate metabolite with 20-fold less inhibitory activity than the final product BS. Incomplete conversion of LBS to BS at a variable efficiency ranging from 10% to 90% was observed either in the native strain S. griseochromogenes or a heterologous producer Streptomyces lividans WJ2. In this study, we determined that maturation of BS from LBS is not a spontaneous process but is governed by a standalone peptidase PepN, which hydrolyzes LBS in a pH-sensitive way with most appropriate of pH 7~8 but is inactive when the pH is below 5 or above 10. PepN1 and PepN2, two neighboring PepN homologs from Streptomyces lividans were purified in E. coli but displayed ca.100-fold difference in LBS hydrolytic activity. Overexpression of pepN1 in WJ2 enhanced BS yield by 100% and lowered the ratio of LBS to BS from 2:1 to 2:3. This work presents the expansion of the biological role for PepN in antibiotic maturation and the first report of hydrolysis of beta amide linkage by this conserved enzyme.