Donia, Mohamed S.

A little help from a friend The Hawaiian sea slug Elysia rufescens grazes on an alga called Bryopsis sp. The alga defends itself from predators using peptide toxins decorated with fatty acids, called kahalalides. Zan et al. wondered if a third party was involved in toxin production (see the Perspective by Mascuch and Kubanek). Within the alga, a species of bacterium with a very reduced genome was discovered to be a factory for the nonribosomal assembly of a family of kahalalides. The authors elucidated the pathways for generating this chemical diversity. It seems that the sea slug not only tolerates the toxins but, to protect itself from being eaten by fish, grazes on the alga to accumulate kahalalide. Science , this issue p. eaaw6732 ; see also p. 1034

Secondary metabolites are ubiquitous in bacteria, but by definition, they are thought to be nonessential. Highly toxic secondary metabolites such as patellazoles have been isolated from marine tunicates, where their exceptional potency and abundance implies a role in chemical defense, but their biological source is unknown. Here, we describe the association of the tunicate Lissoclinum patella with a symbiotic α-proteobacterium, Candidatus Endolissoclinum faulkneri, and present chemical and biological evidence that the bacterium synthesizes patellazoles. We sequenced and assembled the complete Ca . E. faulkneri genome, directly from metagenomic DNA obtained from the tunicate, where it accounted for 0.6% of sequence data. We show that the large patellazoles biosynthetic pathway is maintained, whereas the remainder of the genome is undergoing extensive streamlining to eliminate unneeded genes. The preservation of this pathway in streamlined bacteria demonstrates that secondary metabolism is an essential component of the symbiotic interaction.