Screening of antibacterial activity of Goniothalamus calvicarpa extracts against Xanthomonas axonopodis pv. citri in vitro

Citrus canker caused by Xanthomonas axonopodis pv. citri (Xac) is a serious disease in many parts of the world and is difficult to control. The use of copper-based pesticides is becoming a concern due to the accumulation of heavy metals in orchard soils and more benign treatment methods are needed. Eighteen Thai native plant extracts were screened for antibacterial activity against a Thai isolate of Xac and it was found that ethanolic extracts of Goniothalamus calvicarpa leaves showed the strongest antibacterial activity against Xac in vitro. The G. calvicarpa extracts were then sequentially dissolved with hexane, ethyl acetate and methanol and retested. A 70% aqueous ethanol extract and a methanol soluble extract produced strong inhibition zones against Xac. Although thin layer chromatographic profiles revealed the likely presence of flavonoids in the biologically active extracts of G. calvicarpa, the active compounds have yet to be identified. Work is proceeding to determine whether specific extracts of G. calvicarpa have biological activity against citrus canker in the field

Novel genomic island modifies DNA with 7-deazaguanine derivatives

The discovery of ∼20-kb gene clusters containing a family of paralogs of tRNA guanosine transglycosylase genes, called tgtA5, alongside 7-cyano-7-deazaguanine (preQ[subscript 0]) synthesis and DNA metabolism genes, led to the hypothesis that 7-deazaguanine derivatives are inserted in DNA. This was established by detecting 2’-deoxy-preQ[subscript 0] and 2’-deoxy-7-amido-7-deazaguanosine in enzymatic hydrolysates of DNA extracted from the pathogenic, Gram-negative bacteria Salmonella enterica serovar Montevideo. These modifications were absent in the closely related S. enterica serovar Typhimurium LT2 and from a mutant of S. Montevideo, each lacking the gene cluster. This led us to rename the genes of the S. Montevideo cluster as dpdA-K for 7-deazapurine in DNA. Similar gene clusters were analyzed in ∼150 phylogenetically diverse bacteria, and the modifications were detected in DNA from other organisms containing these clusters, including Kineococcus radiotolerans, Comamonas testosteroni, and Sphingopyxis alaskensis. Comparative genomic analysis shows that, in Enterobacteriaceae, the cluster is a genomic island integrated at the leuX locus, and the phylogenetic analysis of the TgtA5 family is consistent with widespread horizontal gene transfer. Comparison of transformation efficiencies of modified or unmodified plasmids into isogenic S. Montevideo strains containing or lacking the cluster strongly suggests a restriction–modification role for the cluster in Enterobacteriaceae. Another preQ[subscript 0] derivative, 2’-deoxy-7-formamidino-7-deazaguanosine, was found in the Escherichia coli bacteriophage 9g, as predicted from the presence of homologs of genes involved in the synthesis of the archaeosine tRNA modification. These results illustrate a deep and unexpected evolutionary connection between DNA and tRNA metabolism.Deutsche ForschungsgemeinschaftSingapore-MIT Alliance in Research and Technology (SMART

Nematicidal activity of fervenulin isolated from a nematicidal actinomycete, Streptomyces sp. CMU-MH021, on Meloidogyne incognita

An isolate of the actinomycete, Streptomyces sp. CMU-MH021 produced secondary metabolites that inhibited egg hatch and increased juvenile mortality of the root-knot nematode Meloidogyne incognita in vitro. 16S rDNA gene sequencing showed that the isolate sequence was 99% identical to Streptomyces roseoverticillatus. The culture filtrates form different culture media were tested for nematocidal activity. The maximal activity against M. incognita was obtained by using modified basal (MB) medium. The nematicidal assay-directed fractionation of the culture broth delivered fervenulin (1) and isocoumarin (2). Fervenulin, a low molecular weight compound, shows a broad range of biological activities. However, nematicidal activity of fervenulin was not previously reported. The nematicidal activity of fervenulin (1) was assessed using the broth microdilution technique. The lowest minimum inhibitory concentrations (MICs) of the compound against egg hatch of M. incognita was 30 μg/ml and juvenile mortality of M. incognita increasing was observed at 120 μg/ml. Moreover, at the concentration of 250 μg/ml fervenulin (1) showed killing effect on second-stage nematode juveniles of M. incognita up to 100% after incubation for 96 h. Isocoumarin (2), another bioactive compound produced by Streptomyces sp. CMU-MH021, showed weak nematicidal activity with M. incognita