Construction of a genetically improved Streptomyces clavuligerus strain that might have the potential for a larger clavulanic acid production

Abstract

ABSTRACT : Streptomyces clavuligerus (S. clavuligerus) is a Gram-positive soil bacterium capable of producing several β-lactam metabolites, including the β-lactamase inhibitor clavulanic acid (CA). CA effectively inhibits the activity of β-lactamase enzymes, qualifying it as responsible for most of the resistance developed towards β-lactam antibiotics. Despite the significant number of studies for submerged cultivations related to CA production using S. clavuligerus, low titers (~1 g.L-1) rendered low yields obtained when using a wild-type strain. In contrast, more than 7 g.L-1 are produced using genetically modified strains. Furthermore, regulatory mechanisms and pathways governing CA biosynthesis are not fully depicted and characterized for S. clavuligerus. Therefore, the implementation of novel strategies for understanding and improving CA biosynthesis in S. clavuligerus is required. In this study the effect of overexpression of two regulatory genes involved in CA biosynthesis pathway were separately assessed. The claR gene was overexpressed under the control of the constitutive promoter ermE* in the wild type S. clavuligerus strain. Transformant S. clavuligerus/pICLAR obtained via chromosomal integration, increased CA production approximately 1.4-fold using ISP and GSPG media. No morphological differences were observed between S. clavuligerus/pICLAR and control strain, though, we did not evaluate possible mRNA expression differences. Furthermore, the effect of orf21 overexpression on CA production was analyzed for wild type S. clavuligerus strain. The role of orf21 on CA biosynthesis has been previously studied; nevertheless, its performance as CA production regulator is not fully understood. The findings obtained in this study suggest that the role of orf21 on CA biosynthesis is strongly determined by the environmental conditions of the fermentation. As for GSPG, a defined medium, S. clavuligerus/pIORF21 increased CA production 2.6-fold. Likewise, the orf21 overexpression in GSPG stimulated expression of the late CA pathway genes gcas and the genetic regulator adpA, as determined by real-time PCR. In contrast, for ISP, a soy protein based medium, the transformant S. clavuligerus/ pIORF21 decreased CA production by 1.8-fold compared to the control strain. The orf21 overexpression using ISP medium negatively modulates CA biosynthesis, possibly repressing indirectly the expression of the regulatory gene, claR.