Overexpression of metK shows different effects on avermectin production in various Streptomyces avermitilis strains.

The S-adenosylmethionine synthetase gene (metK) from Streptomyces avermitilis was cloned into multi-copy vector pIJ653 and integrative vector pSET152 yielding two metK expression plasmids pYJ02 and pYJ03, respectively. When wild-type strain ATCC31267 was transformed with these two plasmids, avermectin production was increased about 2.0-fold and 5.5-fold, respectively. The introduction of integrative expression plasmid pYJ03 into the engineered strain GB-165, which produces only avermectin B, promoted the production of avermectin approximately 2.0-fold. However, introduction of pYJ02 did not influence avermectin accumulation in GB-165. Moreover, transformation of the avermectin-overproducing industry strain 76-05 with these two plasmids did not stimulate avermectin production. These results showed that there were different effects of metK expression levels on avermectin production in various S. avermitilis strains. Additionally, the transcript levels of metK, aveR (the avermectin pathway-specific regulatory gene) and aveA1 (one avermectin biosynthesis gene) meet the expectation of fermentation levels of avermectin in wild-type strain and its recombinant strains. The gene expression levels of metK, aveR and aveA1 in GB-165 and 76-05 were much higher then those in wild-type strain, which probably limited the increasement of avermectin by overexpression of metK.