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As a kind of biosurfactants, iturin A has attracted people's wide attentions due to their features of biodegradability, environmentally friendly, etc.; however, high production cost limited its extensive application, and the aim of this research wants to improve iturin A production in Bacillus amyloliquefaciens. Firstly, dual promoter was applied to strengthen iturin A synthetase expression, and its yield was increased to 1.25 g/L. Subsequently, original 5'-UTRs of downstream genes (ituA, ituB, and ituC) in iturin A synthetase cluster were optimized, which significantly increased mRNA secondary stability, and iturin A yield produced by resultant strain HZ-T3 reached 2.32 g/L. Secondly, synthetic pathway of α-glucosidase inhibitor 1-deoxynojirimycin was blocked to improve substrate corn starch utilization, and iturin A yield was increased by 34.91% to 3.13 g/L. Thirdly, efficient precursor (fatty acids, Ser, and Pro) supplies were proven as the critical role in iturin A synthesis, and 5.52 g/L iturin A was attained by resultant strain, through overexpressing yngH, serC, and introducing ocD. Meanwhile, genes responsible for poly-γ-glutamic acid, extracellular polysaccharide, and surfactin syntheses were deleted, which led to a 30.98% increase of iturin A yield. Finally, lipopeptide transporters were screened, and iturin A yield was increased by 17.98% in SwrC overexpression strain, reached 8.53 g/L, which is the highest yield of iturin A ever reported. This study laid a foundation for industrial production and application development of iturin A, and provided the guidance of metabolic engineering breeding for efficient production of other metabolites synthesized by non-ribosomal peptide synthetase. KEY POINTS: • Optimizing 5'-UTR is an effective tactics to regulate synthetase cluster expression. • Blocking 1-DNJ synthesis benefited corn starch utilization and iturin A production. • The iturin A yield attained in this work was the highest yield reported so far., (© 2024. The Author(s).)

Avermectins (AVEs) are economically potent anthelmintic agents produced by Streptomyces avermitilis. Among eight AVE components, B1a exhibits the highest insecticidal activity. The purpose of this study was to enhance B1a production, particularly in the high-yielding industrial strain A229, by a combination strategy involving the following steps. (i) aveC gene was engineered to increase B1a:B2a ratio. Three aveC variants (aveC2m, aveC5m, and aveC8m, respectively encoding two, five, and eight amino acid mutations) were synthesized by fusion PCR. B1a:B2a ratio in A229 derivative having kasOp*-controlled aveC8m reached 1.33 (B1a and B2a titers were 8120 and 6124 μg/mL). Corresponding values in A229 were 0.99 and 6447 and 6480 μg/mL. (ii) β-oxidation pathway genes fadD and fadAB were overexpressed in wild-type (WT) strain and A229 to increase supply of acyl-CoA precursors for AVE production. The resulting strains all showed increased B1a titer. Co-overexpression of pkn5p-driven fadD and fadAB in A229 led to B1a titer of 8537 μg/mL. (iii) Genes bicA and ecaA involved in cyanobacterial CO 2 -concentrating mechanism (CCM) were introduced into WT and A229 to enhance carboxylation velocity of acetyl-CoA and propionyl-CoA carboxylases, leading to increased supply of malonyl- and methylmalonyl-CoA precursors and increased B1a titer. Co-expression of bicA and ecaA in A229 led to B1a titer of 8083 μg/mL. (iv) aveC8m, fadD-fadAB, and bicA-ecaA were co-overexpressed in A229, resulting in maximal B1a titer (9613 μg/mL; 49.1% increase relative to A229). Our findings demonstrate that the combination strategy we provided here is an efficient approach for improving B1a production in industrial strains.Key points• aveC mutation increased avermectin B1a:B2a ratio and B1a titer.• Higher levels of acyl-CoA precursors contributed to enhanced B1a production.• B1a titer in an industrial strain was increased by 49.1% via a combination strategy., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Neomycin, an aminoglycoside antibiotic, is widely used in the livestock husbandry due to its higher antimicrobial activity and availability of feed additives in animals. However, its production yield is relatively low and cannot meet the needs of developing market and clinical application. Here, the entire natural neo cluster was cloned from Streptomyces fradiae CGMCC 4.576 by φBT1 integrase-mediated site-specific recombination. Then, the rational reconstruction of the neo cluster was performed by using λ-Red-mediated PCR targeting for improving neomycin production. In order to coordinate with this attempt, the supplementation of suitable precursors was carried out. The constructed recombinant strain Sf/pKCZ03 has multi-copy of the neo cluster modified by disrupting the negative regulatory gene neoI and replacing the native promoter of the neoE-D with P kasO* . Compared to the yield (1282 mg/L) of Streptomyces fradiae CGMCC 4.576, the engineered strain Sf/pKCZ03 had a 36% enhancement of neomycin production. Quantitative real-time PCR analysis revealed the increased transcription of structural genes (neoE, neoB, neoL, aacC8) and regulatory genes (neoR, neoH) in Sf/pKCZ03. Additionally, under the supplementation of 1 g/L N-acetyl-D-glucosamine and 5 g/L L-glutamine, the yield of engineered strain Sf/pKCZ03 showed 62% and 107% improvements compared to that of the wild-type strain in the original medium, respectively. These findings demonstrated that engineering the antibiotic gene cluster in combination with precursors feeding was an effective approach for strain improvement, and would be potentially extended to other Streptomyces for large-scale production of commercialized antibiotics.