Enhancing Ristomycin A Production by Overexpression of ParB-Like StrR Family Regulators Controlling the Biosynthesis Genes.

sp. strain TNS106 harbors a ristomycin-biosynthetic gene cluster () in its genome and produces ristomycin A. Deletion of the sole cluster-situated StrR family regulatory gene, , abolished ristomycin A production and the transcription of the genes to . The ristomycin A fermentation titer in sp. strain TNS106 was dramatically improved by overexpression of and a heterologous StrR family regulatory gene, , from the balhimycin-biosynthetic gene cluster (BGC) utilizing strong promoters and multiple gene copies. Ristomycin A production was improved by approximately 60-fold, resulting in a fermentation titer of 4.01 g/liter in flask culture, in one of the engineered strains. Overexpression of AsrR and Bbr upregulated transcription of tested biosynthetic genes, indicating that these genes were positively regulated by AsrR and Bbr. However, only the promoter region of the operon and the intergenic region upstream of were bound by AsrR and Bbr in gel retardation assays, suggesting that AsrR and Bbr directly regulated the operon and probably to but no other biosynthetic genes. Further assays with synthetic short probes showed that AsrR and Bbr specifically bound not only probes containing the canonical inverted repeats but also a probe with only one 7-bp element of the inverted repeats in its native context. AsrR and Bbr have an N-terminal ParB-like domain and a central winged helix-turn-helix DNA-binding domain. Site-directed mutations indicated that the N-terminal ParB-like domain was involved in activation of ristomycin A biosynthesis and did not affect the DNA-binding activity of AsrR and Bbr. This study showed that overexpression of either a native StrR family regulator (AsrR) or a heterologous StrR family regulator (Bbr) dramatically improved ristomycin A production by increasing the transcription of biosynthetic genes directly or indirectly. The conserved ParB-like domain of AsrR and Bbr was demonstrated to be involved in the regulation of BGC expression. These findings provide new insights into the mechanism of StrR family regulators in the regulation of glycopeptide antibiotic biosynthesis. Furthermore, the regulator overexpression plasmids constructed in this study could serve as valuable tools for strain improvement and genome mining for new glycopeptide antibiotics. In addition, ristomycin A is a type III glycopeptide antibiotic clinically used as a diagnostic reagent due to its side effects. The overproduction strains engineered in this study are ideal materials for industrial production of ristomycin A.