High frequency transformation of Streptomyces niveus protoplasts by plasmid DNA

NovG, a DNA-binding protein acting as a positive regulator of novobiocin biosynthesis

The biosynthetic gene cluster of the aminocoumarin antibiotic novobiocin contains two putative regulatory genes, i.e. novE and novG. The predicted gene product of novG shows a putative helix–turn–helix DNA-binding motif and shares sequence similarity with StrR, a well-studied pathway-specific transcriptional activator of streptomycin biosynthesis. Here functional proof is provided, by genetic and biochemical approaches, for the role of NovG as a positive regulator of novobiocin biosynthesis. The entire novobiocin cluster of the producer organism Streptomyces spheroides was expressed in the heterologous host Streptomyces coelicolor M512, and additional strains were produced which lacked the novG gene within the heterologously expressed cluster. These ΔnovG strains produced only 2 % of the novobiocin formed by the S. coelicolor M512 strains carrying the intact novobiocin cluster. The production could be restored by introducing an intact copy of novG into the mutant. The presence of novG on a multicopy plasmid in the strain containing the intact cluster led to almost threefold overproduction of the antibiotic, suggesting that novobiocin biosynthesis is limited by the availability of NovG protein. Furthermore, purified N-terminal His6-tagged NovG showed specific DNA-binding activity for the novG–novH and the cloG–cloY intergenic regions of the novobiocin and clorobiocin biosynthetic gene clusters, respectively. By comparing the DNA sequences of the fragments binding NovG, conserved inverted repeats were identified in both fragments, similar to those identified as the binding sites for StrR. The consensus sequence for the StrR and the putative NovG binding sites was GTTCRACTG(N)11CRGTYGAAC. Therefore, NovG and StrR apparently belong to the same family of DNA-binding regulatory proteins.

Production of 8′-Halogenated and 8′-Unsubstituted Novobiocin Derivatives in Genetically Engineered Streptomyces coelicolor Strains

In the present study, we produced a hybrid antibiotic, carrying a chlorine atom instead of a methyl group at position 8 of the aminocoumarin moiety of novobiocin. This compound was not accessible by conventional gene inactivation/gene expression experiments due to difficulties in the genetic manipulation of the novobiocin producer Streptomyces spheroides. However, the desired compound was obtained after modification of the novobiocin biosynthetic gene cluster by using lambda-Red-mediated recombination in Escherichia coli, followed by integration of the resulting modified cosmid into the phiC31 attachment site of Streptomyces coelicolor and coexpression of the halogenase Clo-hal of clorobiocin biosynthesis. The halogenase BhaA, responsible for chlorination of tyrosyl moieties of the glycopeptide antibiotic balhimycin, was unable to functionally replace the halogenase Clo-hal, suggesting that the two enzymes have different substrate specificities.

Identification of the novobiocin biosynthetic gene cluster of Streptomyces spheroides NCIB 11891

The novobiocin biosynthetic gene cluster from Streptomyces spheroides NCIB 11891 was cloned by using homologous deoxynucleoside diphosphate (dNDP)-glucose 4,6-dehydratase gene fragments as probes. Double-stranded sequencing of 25.6 kb revealed the presence of 23 putative open reading frames (ORFs), including the gene for novobiocin resistance, gyrB(r), and at least 11 further ORFs to which a possible role in novobiocin biosynthesis could be assigned. An insertional inactivation experiment with a dNDP-glucose 4, 6-dehydratase fragment resulted in abolishment of novobiocin production, since biosynthesis of the deoxysugar moiety of novobiocin was blocked. Heterologous expression of a key enzyme of novobiocin biosynthesis, i.e., novobiocic acid synthetase, in Streptomyces lividans TK24 further confirmed the involvement of the analyzed genes in the biosynthesis of the antibiotic.