Zincophorin - biosynthesis in Streptomyces griseus and antibiotic properties

Abstract

Zincophorin is a polyketide antibiotic that possesses potent activity against Gram-positive bacteria, including human pathogens. While a number of total syntheses of this highly functionalized natural product were reported since its initial discovery, the genetic basis for the biosynthesis of zincophorin has remained unclear. In this study, the co-linearity inherent to polyketide pathways was used to identify the zincophorin biosynthesis gene cluster in the genome of the natural producer Streptomyces griseus HKI 0741. Interestingly, the same locus is fully conserved in the streptomycin-producing actinomycete S. griseus IFO 13350, suggesting that the latter bacterium is also capable of zincophorin biosynthesis. Biological profiling of zincophorin revealed a dose-dependent inhibition of the Gram-positive bacterium Streptococcus pneumoniae. The antibacterial effect, however, is accompanied by cytotoxicity. Antibiotic and cytotoxic activities were completely abolished upon esterification of the carboxylic acid group in zincophorin.

Keywords: Streptococcus pneumoniae; Streptomyces griseus; antibiotic; griseochelin; polyketide; zincophorin.

Table 1. PKS assembly line for the biosynthesis of zincophorin

Table 2. Antibacterial activity of zincophorin, daptomycin, and imipenem. Mean MIC and MBIC values in μM with standard deviation determined for three reference strains and four clinical isolates of S. pneumoniae in microtiter broth dilution and biofilm assay, respectively, are shown.

Table 3. Cytotoxicity of zincophorin, zincophorin methyl ester, daptomycin, and imipenem. Mean values of the 50% cytotoxic concentrations in µM (CC₅₀) with standard deviation are shown.

Figure 1. Structures of zincophorin (R=H) and its methyl ester (R=CH₃).

Figure 2. (A) Organization of the zincophorin biosynthesis gene cluster. (B) Molecular assembly line deduced from zinA to zinG and proposed biosynthesis of zincophorin via the intermediate pre-zincophorin. The acyl-CoA-derived C₂ and C₃ building blocks are highlighted in bold. The domain notation is given in Table 1.

Figure 3. Dose dependence of the effect of zincophorin on growth and biofilm formation of S. pneumoniae DSM20566. Mean values of the inhibitory effect in % with standard deviation of at least 3 tests each with 2 parallels are shown.

Figure 4. MultAlin [51] sequence alignment of ZinJ with the epoxide hydrolases SalBIII and IdmP-Cyc from salinomycin and indanomycin biosynthesis [45], [46]. Selected amino acid residues of the active site cavity [44] are indicated with arrows.