Eva Mösker

Paenilamicin: Structure and Biosynthesis of a Hybrid Nonribosomal Peptide/Polyketide Antibiotic from the Bee Pathogen Paenibacillus larvae

The spore-forming bacterium Paenibacillus larvae is the causative agent of American Foulbrood (AFB), a fatal disease of honey bees that occurs worldwide. Previously, we identified a complex hybrid nonribosomal peptide/polyketide synthesis (NRPS/PKS) gene cluster in the genome of P. larvae. Herein, we present the isolation and structure elucidation of the antibacterial and antifungal products of this gene cluster, termed paenilamicins. The unique structures of the paenilamicins give deep insight into the underlying complex hybrid NRPS/PKS biosynthetic machinery. Bee larval co-infection assays reveal that the paenilamicins are employed by P. larvae in fighting ecological niche competitors and are not directly involved in killing the bee larvae. Their antibacterial and antifungal activities qualify the paenilamicins as attractive candidates for drug development.

Synthesis and structural characterization of hexacoordinate silicon, germanium, and titanium complexes of the E. coli siderophore enterobactin.

The E. coli siderophore enterobactin, one of the strongest Fe(III) chelators known to date, is also capable of binding Si(IV) under physiological conditions. We report on the synthesis and structural characterization of the tris(catecholate) Si(IV) -enterobactin complex and its Ge(IV) and Ti(IV) analogues. Comparative structural analysis, supported by quantum-chemical calculations, reveals the correlation between the ionic radius and the structural changes in enterobactin upon complexation.

The E. coli Siderophores Enterobactin and Salmochelin Form Six‐Coordinate Silicon Complexes at Physiological pH

Iron is essential for nearly all organisms, because it is a key component of many metalloenzymes that catalyze redox reactions of critical importance for cellular growth. Typically, Fe concentrations of 10 –10 m are required for growth of most bacterial species, but under aerobic conditions Fe is not readily bioavailable because of the formation of poorly water-soluble polymeric iron aquo-hydroxo complexes. As a result, the concentration of soluble iron is as low as 10 m at pH 7.4. To extract iron from the environment, bacteria and fungi produce low-molecular-weight chelators, termed siderophores, which possess high Fe affinity. The chelating moieties are typically catechol, hydroxamate, and carboxylate groups. Among them, enterobactin (Ent; 1), produced by E. coli and Salmonella, exhibits the highest binding constant observed thus far. In humans, iron is found in iron-binding proteins, such as transferrin and ferritin, and is a central constituent of myoglobin, hemoglobin, and P450-type monooxygenases. In all cases the iron complex formation constants are orders of magnitude lower than what is observed in bacterial siderophores. Consequently, protein-bound iron in humans can be extracted by siderophores which are therefore considered bacterial virulence factors. The twofold C-glycosylated enterobactin salmochelin (2, Scheme 1), isolated from uropathogenic E. coli and Salmonella enterica, was recently characterized. Surprisingly, we have now found that enterobactin and salmochelin bind Si with high affinity to afford the first examples of silicon complexes of natural products that are stable under physiological conditions. Moreover, our study suggests that Si forms a six-coordinate complex with octahedral geometry.

Lichenicidin Biosynthesis in Escherichia coli: licFGEHI Immunity Genes Are Not Essential for Lantibiotic Production or Self-Protection

ABSTRACT This study demonstrated, for the first time, that immunity genes licFGEHI are not essential for self-protection and production of the two-component lantibiotic lichenicidin in the Gram-negative heterologous host Escherichia coli BLic5. Additionally, it was experimentally demonstrated that lichenicidin lantibiotics are active against the E. coli imp4213 strain, a mutant strain possessing a permeable outer membrane.

A Self‐Sacrificing N‐Methyltransferase Is the Precursor of the Fungal Natural Product Omphalotin

Research on ribosomally synthesized and posttranslationally modified peptides (RiPPs) has led to an increasing understanding of biosynthetic mechanisms, mostly drawn from bacterial examples. In contrast, reports on RiPPs from fungal producers, apart from the amanitins and phalloidins, are still scarce. The fungal cyclopeptide omphalotin A carries multiple N-methylations on the peptide backbone, a modification previously known only from nonribosomal peptides. Mining the genome of the omphalotin-producing fungus for a precursor peptide led to the identification of two biosynthesis genes, one encoding a methyltransferase OphMA that catalyzes the automethylation of its C-terminus, which is then released and cyclized by the protease OphP. Our findings suggest a novel biosynthesis mechanism for a RiPP in which a modifying enzyme bears its own precursor peptide.

Culturable bioactive actinomycetes from the Great Indian Thar Desert

The present study was attempted to determine the antimicrobial potential of actinomycetes from the Great Indian Thar Desert. A total of 100 different morphotypes based on phenotypic characterization were isolated from desert ecosystems located in the northwest of India and tested for their antimicrobial activity by the cross-streak method. Among the strains tested, 13 actinomycetes exhibiting strong antimicrobial activities against several test organisms, including multidrug resistant bacteria (MRSA) were chosen for a phylogenetic diversity study. The results of 16S rRNA gene sequencing showed their affiliation to actinobacterial genera: Actinomadura, Nocardia, Nonomuraea, Spirillispora, and Streptomyces. Three of these isolates were considered to be new members of the Streptomyces genus and another strain also seemed to be a new species of the genus Spirillispora. Among these strains, five were chosen to study the bioactive products using Q-Tof-MS because of their broad spectrum activity against the panel of test pathogens used. The results showed that they produce many known compounds and might produce few unknown compounds as well. This is the first such report on the selective isolation of actinomycetes from the Great Indian Thar desert, and their screening for antibacterial potential. This ecosystem has never before been explored to this extent.