Overexpression of ribosome recycling factor causes increased production of avermectin in Streptomyces avermitilis strains

Ribosome recycling factor (RRF), encoded by frr gene, is involved in the release of ribosomes from the translational post-termination complex for a new round of initiation. In this study, the frr gene with either its own promoter or with ermE*p was cloned into a multi-copy vector, pKC1139, and a single-site integrative vector, pSET152, respectively. The resulting plasmids were transformed into Streptomyces avermitilis wild-type strain ATCC31267, avermectin high-producing mutant strain 76-02-e, and the engineered strain GB-165 that produces only avermectin B. The results showed that overexpression of frr increased avermectin yield (by 3- to 3.7-fold in the wild-type strain) and revealed an frr gene copy number effect; i.e., multiple copies of frr had a greater promoting effect on avermectin production than a single copy in each of the three transformed S. avermitilis strains. Comparison of the growth and expression of the ave genes in an frr-overexpressing strain and wild-type ATCC31267 indicated that frr overexpression promoted cell growth as well as the expression of ave genes (including pathway-specific positive regulatory gene aveR for avermectin biosynthesis and ave structural genes), leading in turn to avermectin overproduction. These findings provide an effective approach for the improvement of antibiotic production in Streptomyces.