Phosphate effect on filipin production and morphological differentiation in Streptomyces filipinensis and the role of the PhoP transcription factor

The biosynthesis of the antifungal filipin in Streptomyces filipinensis is very sensitive to phosphate regulation. Concentrations as low as 2.5 mM block filipin production. This effect is, at least in part, produced by repression of the transcription of most filipin biosynthetic genes. The role of the two-component PhoRP system in this process was investigated. The phoRP system of S. filipinensis was cloned and transcriptionally characterised. PhoP binds to two PHO boxes present in one of its two promoters. Filipin production was greatly increased in ΔphoP and ΔphoRP mutants, in agreement with a higher transcription of the fil genes, and the effect of phosphate repression on the antibiotic production of these strains was significantly reduced. No PhoP binding was observed by electrophoretic mobility gel shift assays (EMSAs) with the promoter regions of the fil gene cluster thus suggesting an indirect effect of mutations. Binding assays with cell-free extracts from the wild-type and mutant strains on fil genes promoters revealed retardation bands in the parental strain that were absent in the mutants, thus suggesting that binding of the putative transcriptional regulator or regulators controlled by PhoP was PhoP dependent. Noteworthy, PhoP or PhoRP deletion also produced a dramatic decrease in sporulation ability, thus indicating a clear relationship between the phosphate starvation response mediated by PhoP and the sporulation process in S. filipinensis. This effect was overcome upon gene complementation, but also by phosphate addition, thus suggesting that alternative pathways take control in the absence of PhoRP.