Pleiotropic Control of Secondary Metabolism and Morphological Development by KsbC, a Butyrolactone Autoregulator Receptor Homologue in Kitasatospora setae

ABSTRACTThe γ-butyrolactone autoregulator signaling cascades have been shown to control secondary metabolism and/or morphological development among manyStreptomycesspecies. However, the conservation and variation of the regulatory systems among actinomycetes remain to be clarified. The genome sequence ofKitasatospora setae, which also belongs to the familyStreptomycetaceaecontaining the genusStreptomyces, has revealed the presence of three homologues of the autoregulator receptor: KsbA, which has previously been confirmed to be involved only in secondary metabolism; KsbB; and KsbC. We describe here the characterization ofksbC, whose regulatory cluster closely resembles theStreptomyces virginiae barAlocus responsible for the autoregulator signaling cascade. Deletion of the geneksbCresulted in lowered production of bafilomycin and a defect of aerial mycelium formation, together with the early and enhanced production of a novel β-carboline alkaloid named kitasetaline. A putative kitasetaline biosynthetic gene cluster was identified, and its expression in a heterologous host led to the production of kitasetaline together with JBIR-133, the production of which is also detected in theksbCdisruptant, and JBIR-134 as novel β-carboline alkaloids, indicating that these genes were biosynthetic genes for β-carboline alkaloid and thus are the first such genes to be discovered in bacteria.

The autoregulator receptor homologue AvaR3 plays a regulatory role in antibiotic production, mycelial aggregation and colony development of Streptomyces avermitilis

The γ-butyrolactone autoregulator receptor has been shown to control secondary metabolism and/or morphological differentiation across many Streptomyces species. Streptomyces avermitilis produces an important anthelmintic agent (avermectin) and two further polyketide antibiotics, filipin and oligomycin. Genomic analysis of S. avermitilis revealed that this micro-organism has the clustered putative autoregulator receptor genes distant from the antibiotic biosynthetic gene clusters. Here, we describe the characterization of avaR3, one of the clustered receptor genes, which encodes a protein containing an extra stretch of amino acid residues that has not been found in the family of autoregulator receptors. Disruption of avaR3 resulted in markedly decreased production of avermectins, with delayed expression of avermectin biosynthetic genes, suggesting that AvaR3 positively controls the avermectin biosynthetic genes. Moreover, the disruption caused increased production of filipin without any changes in the transcriptional profile of the filipin biosynthetic genes, suggesting that filipin production is indirectly controlled by AvaR3. The avaR3 disruptant displayed fragmented growth in liquid culture and conditional morphological defects on solid medium. These findings demonstrated that AvaR3 acts as a global regulator that controls antibiotic production and cell morphology.