Nourseothricins (syn. Streptothricins), agroupof nucleoside peptides produced by several streptomycete strains, contain a polyb-lysine chain of variable length attached in amide linkage to the amino sugar moiety gulosamine of the nucleoside portion. We show that the nourseothricin-pro-ducingStreptomyces nourseicontains an enzyme (NpsA) of an apparentMr56 000 that speci®cally activatesb-lysine by adenylation but does not bind to it as a thioester.