Evidence for a Symbiosis Island Involved in Horizontal Acquisition of Pederin Biosynthetic Capabilities by the Bacterial Symbiont of Paederus fuscipes Beetles

Abstract

Pederin belongs to a group of antitumor compounds found in terrestrial beetles and marine sponges. It is used by apparently all members of the rove beetle genera Paederus and Paederidus as a chemical defense against predators. However, a recent analysis of the putative pederin biosynthesis (ped) gene cluster strongly suggests that pederin is produced by bacterial symbionts. We have sequenced an extended region of the symbiont genome to gain further insight into the biology of this as-yet-unculturable bacterium and the evolution of pederin symbiosis. Our data indicate that the symbiont is a very close relative of Pseudomonas aeruginosa that has acquired several foreign genetic elements by horizontal gene transfer. Besides one functional tellurite resistance operon, the region contains a genomic island spanning 71.6 kb that harbors the putative pederin biosynthetic genes. Several decayed insertion sequence elements and the mosaic-like appearance of the island suggest that the acquisition of the ped symbiosis genes was followed by further insertions and rearrangements. A horizontal transfer of genes for the biosynthesis of protective substances could explain the widespread occurrence of pederin-type compounds in unrelated animals from diverse habitats.The pederin group of natural products consists of numerous members with potent antitumor and antiviral activities ( Fig. 1) (15). Remarkably, these structurally highly similar substances have been isolated from two entirely unrelated animal groups. Pederin itself occurs in species of the beetle genera Paederus and Paederidus, while almost all other members are known only from marine sponges. In the beetles, pederin serves as a chemical defense against predators such as spiders (11). It is also the cause of Paederus dermatitis, a blistering inflammation that is contracted when the beetles are accidentally crushed on the human skin (20). In regions with a warm climate the beetles can appear in enormous numbers, resulting in notorious epidemics that have forced the evacuation of entire settlements (18,25). Interestingly, they have also been proposed as a cause for 3 of the 10 biblical plagues (16).Several lines of evidence clearly indicate that pederin is not produced by the beetles themselves but by as-yet-uncultured bacterial symbionts. Kellner and Dettner have demonstrated that within each examined beetle species pederin is synthesized in only about 90% of the females, who alone can transfer this trait to their offspring (10). According to 16S ribosomal DNA data, only these pederin-synthesizing females harbor a bacterium with the closest relationship to Pseudomonas aeruginosa (8). If eggs of such beetles are fed to nonproducing females, pederin is again synthesized in their offspring (7). Our search for the pederin biosynthesis genes, motivated by the prospect of generating an alternative biotechnological source of these rare antitumor compounds, has resulted in the isolation of a polyketide synthase cluster spanning 54 kb (19). This operon, designated the ped cluster...

Metagenomic natural product discovery in lichen provides evidence for a family of biosynthetic pathways in diverse symbioses

“…The presence of similar trans-AT PKS-NRPS gene clusters in different groups of bacteria has suggested that these clusters are horizontally transferred (44). The flanking of the nsp cluster by transposable elements is consistent with this hypothesis, and the mosaic of homologies across the gene cluster suggests involvement of several intergenomic and intragenomic recombination events.…”

“…2). Furthermore, although the taxonomic identities of their producers are unknown, with the exception of pederin, which is produced by a close relative of Pseudomonas aeruginosa (21,43,44), both lichen metagenomic data and expression and product characterization in Nostoc sp. N6 clearly show that nosperin derives from the cyanobacterial photobiont of P. membranacea.…”

Antitumor polyketide biosynthesis by an uncultivated bacterial symbiont of the marine sponge Theonella swinhoei

“…Further cloning and sequencing of phylogenetically relevant genes outside the PKS system will be necessary to determine whether the symbiont is a member of the genus Pseudomonas as shown for the pederin producer. Because our data indicate that pederin-type genes have been subject to horizontal gene transfer (12), production of the compounds may well be independent from bacterial taxonomy. So far, data about the microbiology of T. swinhoei exist only for a specimen collected at Palau (21).…”

“…The fragmentation of the Pseudomonas sp. ped system into three distinct genome regions is most unusual for bacterial PKS pathways but correlates with the large number of transposase pseudogenes that frame the two previously isolated ped regions (9,11,12).…”

“…1), which they use as chemical defense (10). We isolated a set of putative pederin biosynthesis genes (9,11) from the metagenome of Paederus fuscipes beetles and traced them to a bacterial symbiont with the closest relationship to Pseudomonas aeruginosa (12). The genes encode a mixed modular polyketide synthase (PKS)-nonribosomal peptide synthetase (NRPS) system and were found to be distributed among two distinct regions of the symbiont genome (pedIJK and pedABCDEF in Fig.…”