Mebamamide C, a deoxy analogue of mebamamides in Bryopsis marine green algae and Elysia sacoglossan mollusks.

Kahalalides, originally isolated from the sacoglossan mollusk Elysia rufescens, have been found in various Elysia and Bryopsis species, with over 20 variants identified to date. These compounds are biosynthesized by Candidatus Endobryopsis kahalalidefaciens within Bryopsis species. In this study, we report the isolation and structural determination of a new cyclic depsipeptide, mebamamide C (1), from Bryopsis sp. The planar structure was determined by spectroscopic data analyses, and the absolute configurations were determined using Marfey's method and modified Mosher's method. Additionally, our study explores the chemical relationship between Bryopsis algae and Elysia mollusks. The individual chemical profiles of these marine organisms highlight a fascinating aspect of marine chemical ecology. The distinct, species-specific chemical profiles observed in Elysia species imply the possibility of a symbiotic relationship with the kahalalide-producing bacteria.

Structure Determination of Kahalalide Analogues Based on Metagenomic Analysis of a Bryopsis sp. Marine Green Alga.

Two kahalalide analogues were isolated from a Bryopsis sp. marine green alga. Even though our initial structure determination of the peptides by NMR and MS identified them as kahalalide Z1 (KZ1; 3) and Z2 (KZ2; 4), the absolute configuration of the Thr residues by Marfey's analysis was different from those found in kahalalide F (KF), 3, and 4. To ascertain the absolute configuration of the amino acid residues genetically, we conducted a metagenomic analysis for symbiotic bacteria in the alga, leading to the biosynthetic gene cluster (BGC) responsible for producing the kahalalides named kahalalides Z3 (KZ3; 1) and Z4 (KZ4; 2). The identification of amino acid residues based on the A-domain suggested these peptides possess the amino acid sequence d-allo-Thr-l-Val-l-Val-d-Val residues at the N-terminus, instead of the d-Val-l-Thr-l-Val-d-Val residues found in KF, 3, and 4. The N-terminal amino acid sequence including absolute configuration was unambiguously determined by a comparison of LCMS data of synthetic tetrapeptides and the hydrolysates derived from 1 and 2. This structural difference is caused by swapping the substrate specificities of the first two A-domains.

Metagenomic Insights Reveal Unrecognized Diversity of Entotheonella in Japanese Theonella Sponges.

Numerous biologically active natural products have been discovered from marine sponges, particularly from Theonella swinhoei, which is known to be a prolific source of natural products such as polyketides and peptides. Recent studies have revealed that many of these natural products are biosynthesized by Candidatus Entotheonella phylotypes, which are uncultivated symbionts within T. swinhoei. Consequently, Entotheonella is considered an untapped biochemical resource. In this study, we conducted metagenomic analyses to assess the diversity of Entotheonella in two T. swinhoei Y and two T. swinhoei W (Y and W referring to the yellow and white interior of the sponge, respectively), after separating filamentous bacteria using density gradient centrifugation. We obtained five Entotheonella metagenome-assembled genomes (MAGs) from filamentous bacteria-enriched fractions. Notably, one of these MAGs is significantly different from previously reported Entotheonella variants. Additionally, we identified closely related Entotheonella members present across different chemotypes of T. swinhoei. Thus, our metagenomic insights reveal that the diversity of Entotheonella within Theonella sponges is greater than previously recognized.