A novel enzyme conferring streptothricin resistance alters the toxicity of streptothricin D from broad-spectrum to bacteria-specific

Abstract

Streptothricins (STs) produced by Streptomyces strains are broad-spectrum antibiotics. All STs consist of a carbamoylated D-gulosamine to which the β-lysine homopolymer (1 to 7 residues) and the amide form of the unusual amino acid streptolidine (streptolidine lactam) are attached. Although many ST-resistance genes have been identified in bacteria, including clinically isolated pathogens and ST-producing Streptomyces strains, only one resistance mechanism has been identified to date. This mechanism involves the modification of the ST molecule by monoacetylation of the moiety of the β-lysine(s). In this study, we successfully isolated a novel ST-resistance gene (sttH) from Streptomyces albulus, which is a known ST non-producer. The in vitro analysis of SttH demonstrated that this enzyme catalyzes the hydrolysis of the amide bond of streptolidine lactam, thereby conferring ST resistance. Interestingly, the selective toxicity of ST-D possessing 3x β-lysine moiety was altered from broad-spectrum to bacteria-specific by the hydrolysis of streptolidine lactam, although ST-F (1x β-lysine) was detoxified by SttH in both prokaryotes and eukaryotes (yeasts). STs have not been clinically developed due to their toxicities; however, in this study, we showed that hydrolyzed ST-D (ST-Dacid) exhibits potent antibacterial activity even when its toxicity against eukaryotic cells is reduced by SttH. This suggests that ST-D-acid is a potential candidate for clinical development or for use as a new lead compound for drug discovery. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.