Zinc-Responsive Regulator Zur Regulates Zinc Homeostasis, Secondary Metabolism, and Morphological Differentiation in Streptomyces avermitilis

Abstract

Biosynthesis of secondary metabolites and morphological differentiation in bacteria are affected by environmental signals. The molecular mechanisms whereby zinc availability affects secondary metabolism and morphological differentiation remain poorly understood.

ABSTRACT

Zinc is an essential cofactor for many metal enzymes and transcription regulators. Zn ²⁺ availability has long been known to affect antibiotic production and morphological differentiation of Streptomyces species. However, the molecular mechanism whereby zinc regulates these processes remains unclear. We investigated the regulatory roles of the zinc-sensing regulator Zur in Streptomyces avermitilis . Our findings demonstrate that Zur plays an essential role in maintaining zinc homeostasis by repressing the expression of the zinc uptake system ZnuACB and alternative non-zinc-binding ribosomal proteins and promoting the expression of zinc exporter ZitB. Deletion of the zur gene resulted in decreased production of avermectin and oligomycin and delayed morphological differentiation, and these parameters were restored close to wild-type levels in a zur -complemented strain. Zur bound specifically to Zur box in the promoter regions of avermectin pathway-specific activator gene aveR , oligomycin polyketide synthase gene olmA1 , and filipin biosynthetic pathway-specific regulatory genes pteR and pteF . Analyses by reverse transcription quantitative PCR and luciferase reporter systems indicated that Zur directly activates the transcription of these genes, i.e., that Zur directly activates biosynthesis of avermectin and oligomycin. Zur positively regulated morphological development by repressing the transcription of differentiation-related genes ssgB and minD2 . Our findings, taken together, demonstrate that Zur in S. avermitilis directly controls zinc homeostasis, biosynthesis of avermectin and oligomycin, and morphological differentiation.

IMPORTANCE Biosynthesis of secondary metabolites and morphological differentiation in bacteria are affected by environmental signals. The molecular mechanisms whereby zinc availability affects secondary metabolism and morphological differentiation remain poorly understood. We identified several new target genes of the zinc response regulator Zur in Streptomyces avermitilis , the industrial producer of avermectin. Zur was found to directly and positively control avermectin production, oligomycin production, and morphological differentiation in response to extracellular Zn ²⁺ levels. Our findings clarify the regulatory functions of Zur in Streptomyces , which involve linking environmental Zn ²⁺ status with control of antibiotic biosynthetic pathways and morphological differentiation.