Enhanced production of antifungal lipopeptide iturin A by Bacillus amyloliquefaciens LL3 through metabolic engineering and culture conditions optimization.

ABSTRACT

BACKGROUND: Iturins, which belong to antibiotic cyclic lipopeptides mainly produced by Bacillus sp., have the potential for application in biomedicine and biocontrol because of their hemolytic and antifungal properties. Bacillus amyloliquefaciens LL3, isolated previously by our lab, possesses a complete iturin A biosynthetic pathway as shown by genomic analysis. Nevertheless, iturin A could not be synthesized by strain LL3, possibly resulting from low transcription level of the itu operon. RESULTS: In this work, enhanced transcription of the iturin A biosynthetic genes was implemented by inserting a strong constitutive promoter C2up into upstream of the itu operon, leading to the production of iturin A with a titer of 37.35 mg l-1. Liquid chromatography-mass spectrometry analyses demonstrated that the strain produced four iturin A homologs with molecular ion peaks at m/z 1044, 1058, 1072 and 1086 corresponding to [C14 + 2H]2+, [C15 + 2H]2+, [C16 + 2H]2+ and [C17 + 2H]2+. The iturin A extract exhibited strong inhibitory activity against several common plant pathogens. The yield of iturin A was improved to 99.73 mg l-1 by the optimization of the fermentation conditions using a response surface methodology. Furthermore, the yield of iturin A was increased to 113.1 mg l-1 by overexpression of a pleiotropic regulator DegQ. CONCLUSIONS: To our knowledge, this is the first report on simultaneous production of four iturin A homologs (C14-C17) by a Bacillus strain. In addition, this study suggests that metabolic engineering in combination with culture conditions optimization may be a feasible method for enhanced production of bacterial secondary metabolites.