Avermectins produced by Streptomyces avermitilis are commercially important anthelmintic agents. The detailed regulatory mechanisms of avermectin biosynthesis remain unclear. Here, we identified SAV3619, a TetR-family transcriptional regulator designated AveT, to be an activator for both avermectin production and morphological differentiation in S. avermitilis. AveT was shown to indirectly stimulate avermectin production by affecting transcription of the cluster-situated activator gene aveR. AveT directly repressed transcription of its own gene (aveT), adjacent gene pepD2 (sav_3620), sav_7490 (designated aveM), and sav_7491 by binding to an 18-bp perfect palindromic sequence (CGAAACGKTKYCGTTTCG, where K is T or G and Y is T or C and where the underlining indicates inverted repeats) within their promoter regions. aveM (which encodes a putative transmembrane efflux protein belonging to the major facilitator superfamily [MFS]), the important target gene of AveT, had a striking negative effect on avermectin production and morphological differentiation. Overexpression of aveT and deletion of aveM in wild-type and industrial strains of S. avermitilis led to clear increases in the levels of avermectin production. In vitro gel-shift assays suggested that C-5-O-B1, the late pathway precursor of avermectin B1, acts as an AveT ligand. Taken together, our findings indicate positive-feedback regulation of aveT expression and avermectin production by a late pathway intermediate and provide the basis for an efficient strategy to increase avermectin production in S. avermitilis by manipulation of AveT and its target gene product, AveM.
Soil-dwelling species of Streptomyces produce about half of currently known antibiotics (including antibacterial, anticancer, anthelmintic, and immunosuppressive agents) during their complex morphological differentiation cycle (1) and have many important medical and commercial applications. Antibiotic biosynthesis is controlled by large gene clusters, usually including cluster-situated regulators (CSRs). These CSRs are at the lowest level of the complex regulatory network for antibiotic biosynthesis and are controlled by various higher-level pleiotropic regulators in response to developmental state, population density, environmental signals, and physiological signals (2-4).The species Streptomyces avermitilis produces avermectins, a series of 16-membered macrocyclic lactones (termed A1a, A1b, A2a, A2b, B1a, B1b, B2a, and B2b) that are excellent anthelmintic agents with high potency, broad-spectrum activity against various arthropod and nematode parasites, and a low level of side effects on the host (5, 6). Of the eight avermectin components, B1a has the highest insecticidal activity (7). Avermectins are a commercially important group of antibiotics with annual worldwide sales of ϳ$850 million (8) and are widely applied in the agricultural, veterinary, and medical fields. The 82-kb ave gene cluster that controls avermectin biosynthesis includes 18 open reading frames (ORFs) (9). The gene aveR, loc...