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Andreas Bechthold

Publications and source records attributed to Andreas Bechthold.

27 records · Page 2Linked to original sources
PubMed2004

Expression of the landomycin biosynthetic gene cluster in a PKS mutant of Streptomyces fradiae is dependent on the coexpression of a putative transcriptional activator gene.

The formation of landomycin A or one of its derivatives (5,6-anhydrolandomycin A) in a heterologous strain has never been achieved. It has now been made possible by the coexpression of a cosmid containing all biosynthetic genes necessary to produce landomycin A together with a pathway-specific regulatory gene. As host we used a polyketide synthase-defective mutant strain of Streptomyces fradiae Tü2717 which is not able to produce urdamycin A. Our results indicate that four glycosyltransferases are responsible for the formation of the hexasaccharide side chain of landomycin A.

Aminoglycosides↗
PubMed2004

Novel avilamycin derivatives with improved polarity generated by targeted gene disruption.

The oligosaccharide antibiotics avilamycin A and C are produced by Streptomyces viridochromogenes Tu57. Both consist of a heptasaccharide chain, which is attached to a polyketide-derived dichloroisoeverninic acid moiety. They show excellent antibiotic activity against Gram-positive bacteria. Both molecules are modified by O-methylation at different positions, which contributes to poor water solubility and difficulties in galenical drug development. In order to generate novel avilamycin derivatives with improved polarity and improved pharmacokinetic properties, we generated a series of mutants with one, two, or three mutated methyltransferase genes. Based on the structure of the novel avilamycin derivatives, the exact function of three methyltransferases, AviG2, AviG5, and AviG6, involved in avilamycin biosynthesis could be assigned.

Gene Targeting↗
PubMed2004

Generation of Streptomyces globisporus SMY622 strain with increased landomycin E production and it's initial characterization.

Landomycin E (LaE) overproducing strain Streptomyces globisporus SMY6222 has been developed using UV induced mutagenesis and selection for streptomycin resistance. SMY622 has been shown by HPLC to produce 200-fold higher amounts of LaE when comparing with parental strain. The levels of transcription of regulatory gene lndI and oxygenase gene lndE are two times higher in the mutant than in the wild type. Gene rpsL for ribosomal protein S12 from SMY622 was shown to contain point mutation K43R. Possible reasons for increased LaE synthesis in SMY622 are discussed.

Aminoglycosides↗
PubMed2003

Crystal structure of the avilamycin resistance-conferring methyltransferase AviRa from Streptomyces viridochromogenes.

The emergence of antibiotic-resistant bacterial strains is a widespread problem in contemporary medical practice and drug design. It is therefore important to elucidate the underlying mechanism in each case. The methyltransferase AviRa from Streptomyces viridochromogenes mediates resistance to the antibiotic avilamycin, which is closely related to evernimicin, an oligosaccharide antibiotic that has been used in medical studies. The structure of AviRa was determined by X-ray diffraction at 1.5A resolution. Phases were obtained from one selenomethionine residue introduced by site-directed mutagenesis. The chain-fold is similar to that of most methyltransferases, although AviRa contains two additional helices as a specific feature. A putative-binding site for the cofactor S-adenosyl-L-methionine was derived from homologous structures. It agrees with the conserved pattern of interacting amino acid residues. AviRa methylates a specific guanine base within the peptidyltransferase loop of the 23S ribosomal RNA. Guided by the target, the enzyme was docked to the cognate ribosomal surface, where it fit well into a deep cleft without contacting any ribosomal protein. The two additional alpha-helices of AviRa filled a depression in the surface. Since the transferred methyl group of the cofactor is in a pocket beneath the enzyme surface, the targeted guanine base has to flip out for methylation.

Amino Acid Sequence↗
PubMed2003

The C-Glycosyltransferase UrdGT2 is unselective toward d- and l-configured nucleotide-bound rhodinoses.

UrdGT2 is a d-olivosyltransferase from the metabolic pathway of urdamycin A, an angucycline antitumor and antimicrobial drug. The remarkable feature of this biocatalyst is its ability to set up C-glycosidic bonds. Using an in vivo system suitable to deliver the trideoxysugar rhodinose in both d- and l- configuration we could verify that both have been accepted as substrates and attached to the urdamycin polyketide backbone via a C-glycosidic bond. Regardless of the stereochemistry, these C-glycosides served as acceptor for a subsequent glycosylation step to yield the novel urdamycins R and S with di-rhodinosyl side chains at C-9 of the polyketide moiety.

Glycosyltransferases↗
PubMed2003

The avilamycin resistance determinants AviRa and AviRb methylate 23S rRNA at the guanosine 2535 base and the uridine 2479 ribose.

Avilamycin is an orthosomycin antibiotic that has shown considerable potential for clinical use, although it is presently used as a growth promoter in animal feed. Avilamycin inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit. The ribosomes of the producer strain, Streptomyces viridochromogenes Tü57, are protected from the drug by the action of three resistance factors located in the avilamycin biosynthetic gene cluster. Two of the resistance factors, aviRa and aviRb, encode rRNA methyltransferases that specifically target 23S rRNA. Recombinant AviRa and AviRb proteins retain their activity after purification, and both specifically methylate in vitro transcripts of 23S rRNA domain V. Reverse transcriptase primer extension indicated that AviRa is an N-methyltransferase that targets G2535 within helix 91 of the rRNA, whereas AviRb modified the 2'-O-ribose position of nucleotide U2479 within helix 89. MALDI mass spectrometry confirmed the exact positions of each of these modifications, and additionally established that a single methyl group is added at each nucleotide. Neither of these two nucleotides have previously been described as a target for enzymatic methylation. Molecular models of the 50S subunit crystal structure show that the N-1 of the G2535 base and the 2'-hydroxyl of U2479 are separated by approximately 10 A, a distance that can be spanned by avilamycin. In addition to defining new resistance mechanisms, these data refine our understanding of the probable ribosome contacts made by orthosomycins and of how these antibiotics inhibit protein synthesis.

Animals↗
PubMed2002

Rationally designed glycosylated premithramycins: hybrid aromatic polyketides using genes from three different biosynthetic pathways.

Heterologous expression of the urdGT2 gene from the urdamycin producer Streptomyces fradiae Tü2717, which encodes a C-glycosyltransferase, into mutants of the mithramycin producer Streptomyces argillaceus, in which either one or all glycosyltransferases were inactivated, yielded four novel C-glycosylated premithramycin-type molecules. Structure elucidation revealed these to be 9-C-olivosylpremithramycinone, 9-C-mycarosylpremithramycinone, and their respective 4-O-demethyl analogues. In another experiment, both the urdGT2 gene from S. fradiae and the lanGT1 gene from S. cyanogenus, were coexpressed into a S. argillaceus mutant lacking the MtmGIV glycosyltransferase. This experiment, in which genes from three different organisms were combined, resulted in the production of 9-C-(olivo-1-4-olivosyl)premithramycinone. These results prove the unique substrate flexibility of the C-glycosyltransferase UrdGT2, which tolerates not only a variety of sugar-donor substrates, but also various acceptor substrates. The five new hybrid products also represent the first compounds, in which sugars were attached to a position that is normally unglycosylated. The successful combination of two glycosyltransferases in the latter experiment proves that the design of saccharide side chains by combinatorial biosynthetic methods is possible.

Antibiotics, Antineoplastic↗
PubMed2002

Engineered urdamycin glycosyltransferases are broadened and altered in substrate specificity.

Combinatorial biosynthesis is a promising technique used to provide modified natural products for drug development. To enzymatically bridge the gap between what is possible in aglycon biosynthesis and sugar derivatization, glycosyltransferases are the tools of choice. To overcome limitations set by their intrinsic specificities, we have genetically engineered the protein regions governing nucleotide sugar and acceptor substrate specificities of two urdamycin deoxysugar glycosyltransferases, UrdGT1b and UrdGT1c. Targeted amino acid exchanges reduced the number of amino acids potentially dictating substrate specificity to ten. Subsequently, a gene library was created such that only codons of these ten amino acids from both parental genes were independently combined. Library members displayed parental and/or a novel specificity, with the latter being responsible for the biosynthesis of urdamycin P that carries a branched saccharide side chain hitherto unknown for urdamycins.

Amino Acid Sequence↗