Overexpression of the ABC transporter AvtAB increases avermectin production in Streptomyces avermitilis

Avermectins are 16-membered macrocyclic polyketides with potent antiparasitic activities, produced by Streptomyces avermitilis. Upstream of the avermectin biosynthetic gene cluster, there is the avtAB operon encoding the ABC transporter AvtAB, which is highly homologous to the mammalian multidrug efflux pump P-glycoprotein (Pgp). Inactivation of avtAB had no effect, but increasing the concentration of avtAB mRNA 30-500-fold, using a multi-copy plasmid in S. avermitilis, enhanced avermectin production about two-fold both in the wild-type and in a high-yield producer strain on agar plates. In liquid industrial fermentation medium, the overall productivity of avermectin B1a in the engineered high-yield producer was improved for about 50%, from 3.3 to 4.8 g/l. In liquid YMG medium, moreover, the ratio of intracellular to extracellular accumulation of avermectin B1a was dropped from 6:1 to 4.5:1 in response to multiple copies of avtAB. Additionally, the overexpression of avtAB did not cause any increased expression of the avermectin biosynthetic genes through RT-PCR analysis. We propose that the AvtAB transporter exports avermectin, and thus reduces the feedback inhibition on avermectin production inside the cell. This strategy may be useful for enhancing the production of other antibiotics.