Identification of the Putative Bryostatin Polyketide Synthase Gene Cluster from “Candidatus Endobugula sertula”, the Uncultivated Microbial Symbiont of the Marine Bryozoan Bugula neritina

Abstract

The bryostatins are protein kinase C modulators with unique structural features and potential anticancer and neurological activities. These complex polyketides were isolated from the marine bryozoan Bugula neritina, but recent studies indicate that they are produced by the uncultured symbiotic bacterium "Candidatus Endobugula sertula" ("E. sertula"). Here we present the putative biosynthetic genes: five modular polyketide synthase (PKS) genes, a discrete acyltransferase, a beta-ketosynthase, a hydroxy-methyl-glutaryl CoA synthase (HMG-CS), and a methyltransferase. The cluster was sequenced in two closely related "E. sertula" strains from different host species. In one strain the gene cluster is contiguous, while in the other strain it is split into two loci, with one locus containing the PKS genes and the other containing the accessory genes. Here, we propose a hypothesis for the biosynthesis of the bryostatins. Thirteen PKS modules form the core macrolactone ring, and the pendent methyl ester groups are added by the HMG-CS gene cassette. The resulting hypothetical compound bryostatin 0 is the common basis for the 20 known bryostatins. As "E. sertula" is to date uncultured, heterologous expression of this biosynthetic gene cluster has the potential of producing the bioactive bryostatins in large enough quantities for development into a pharmaceutical.

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

“…Some polyketides produced by trans-AT PKSs, such as pederin (21,22,43), bryostatin (45,46), and rhizoxin (12,47), are known to participate in host defense and pathogenicity in symbiotic associations. It has also been suggested that PKS-NRPS compounds such as the microcystins, sometimes produced by cyanobacterial symbionts, may contribute to the chemical defense of lichens against grazers (35,36).…”

Desorption electrospray ionization mass spectrometry reveals surface-mediated antifungal chemical defense of a tropical seaweed

“…Recent studies have provided convincing evidence that a number of secondary metabolites originally ascribed to sponges, bryozoans, and other macroorganisms are of microbial biosynthetic origin (44)(45)(46). Further, microbial metabolites such as 2,3-indolinedione from a bacterium associated with crustacean embryos have been shown to defend hosts against pathogen infection (47).…”

Localization of ‘Candidatus Endobugula sertula’ and the bryostatins throughout the life cycle of the bryozoan Bugula neritina

“…Antibiotic treatments, which reduced 'E. sertula' density in B. neritina adult colonies, decreased bryostatin concentration in B. neritina, and lowered expression of bryA, a portion of the bryostatin biosynthetic gene cluster (Davidson et al, 2001;Hildebrand et al, 2004;Sudek et al, 2007). Such antibiotic treatments of B. neritina did not alter the growth, settlement or metamorphosis of B. neritina (Davidson et al, 2001;Lopanik et al, 2004), but the reduction of bryostatins did increase the palatability of the larvae to fish (Lopanik et al, 2004).…”