• Identification and genetics of 6-thioguanine secreted by Erwinia species and its interference with the growth of other bacteria.

      Wensing, A; Gernold, M; Jock, S; Jansen, R; Geider, K (2014-04)
      We identified a compound in culture supernatants of Erwinia species, such as Erwinia amylovora, E. pyrifoliae, E. billingiae, E. tasmaniensis, E. persicina and E. rhapontici absorbing at 340 nm, which was associated before with the yellow pigment produced by E. amylovora on media containing copper ions. The compound was purified from E. tasmaniensis strain Et1/99 supernatants by chromatography on Dowex-1 and Dowex-50 columns and identified by HPLC/MS and NMR analysis as 6-thioguanine (6TG). Its signal at 167 Da matched with the expected molecular mass. By random mutagenesis with miniTn5, we obtained mutants defective in the genes for pyrimidine and purine metabolism. A specific gene cluster with ycf genes described by us before, absent in the corresponding region of Escherichia coli, was identified in the genome sequence of three Erwinia species and named tgs region for thioguanine synthesis. Clones of the tgs gene cluster promoted 6TG synthesis and secretion in E. coli, when the bacteria were grown in minimal medium supplemented with amino acids. 6TG was bacteriostatic for E. coli and Salmonella typhimurium strains, with cell growth resumed after prolonged incubation. Similar results were obtained with P. agglomerans strains. Bacteria from the genus Pectobacterium were barely and Rahnella or Gibbsiella species were not inhibited by 6TG. Adenine and guanine relieved the toxic effect of 6TG on E. coli. Non-producing strains were fully virulent on host plants. 6TG synthesis may help erwinias to interfere with growth of some microorganisms in the environment.
    • Production of Obionin A and Derivatives by the Sooty Blotch Fungus Microcyclospora malicola.

      Surup, Frank; Medjedović, Ajda; Schroers, Hans-Josef; Stadler, Marc; Helmholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig. (2015-10)
      A multitude of sooty blotch and flyspeck fungi, mainly belonging to the Ascomycetes order Capnodiales, causes dark blemishes and flyspeck-like spots on apples worldwide. Different sooty blotch and flyspeck fungi can coexist in the same orchard and even on a single fruit. Our preceding experiments revealed an activity of Microcyclospora malicola strain 1930 against the anthracnose fungus Colletotrichum fioriniae in dual culture assays. Extracts of M. malicola strain 1930 showed a broad bioactivity against filamentous fungus Mucor hiemalis and gram-positive bacterium Bacillus subtilis. A bioactivity-guided isolation led to the identification of obionin A (1) as the main active principle. In addition to 1, which was previously isolated from the marine fungus Leptosphaeria obiones, we isolated three derivatives. Metabolite 2 bears a keto function at C-6, besides the replacement of oxygen by nitrogen at position 10. Two more derivatives are adducts (3, 4) of acetone as work-up artifacts. Because obionin A (1) and its derivative 2 showed cytotoxic effects and antifungal activities, we propose a role of these secondary metabolites in the antagonism between M. malicola and other apple colonizing sooty blotch and flyspeck fungi, other epiphytes, or apple pathogens competing for the same ecological niche.