01:15 PM Philipp Wichmann (AG Erb)

New-to-nature CO₂ assimilation pathway enabled by an engineered B₁₂ dependent mutase

Acetyl-CoA is a key metabolic intermediate and the product of various natural and synthetic one-carbon (C1) assimilation pathways. While an efficient conversion of acetyl-CoA into other central metabolites, such as pyruvate, is imperative for high biomass yields, available aerobic pathways typically release previously fixed carbon in form of CO2. To overcome this loss of carbon, we developed a new-to-nature pathway, the “Lcm module”, in this study. The Lcm module provides a direct link between acetyl-CoA and pyruvate, is shorter than any other oxygen-tolerant route and notably fixes CO2, instead of releasing it. The Lcm module relies on the new-to-nature activity of a coenzyme B12-dependent mutase. We demonstrate activity of the scaffold enzyme and further improve catalytic efficiency 10-fold by combining in vivo targeted hypermutation and adaptive evolution in an engineered Escherichia coli selection strain. Finally, in a proof-of-principle, we demonstrate the complete Lcm module in vitro. Our work provides a synthetic CO2-assimilating acetyl-CoA assimilation route that expands the metabolic solution space of central carbon metabolism, providing new options for synthetic biology and metabolic engineering.

01:45 PM Joao Felipe Moreira Salgado (FG Brune)

Microbial symbiotic degradation of cellulose in the hind gut of termites

The microbial landscape within termite guts varies across termite families. The gut microbiota of lower termites (LT) is dominated by cellulolytic flagellates, which sequester wood particles in their digestive vacuoles, whereas in the flagellate-free higher termites (HT), cellulolytic activity has been attributed to fiber-associated bacteria. However, little is known about the role of individual lineages in fiber digestion, particularly in LT. Here, we investigate the lignocellulolytic potential of 2,223 metagenome-assembled genomes (MAGs) recovered from the gut metagenomes of 51 termite species. In the flagellate-dependent LT, cellulolytic enzymes are restricted to MAGs of Bacteroidota (Dysgonomonadaceae, Tannerellaceae, Bacteroidaceae, Azobacteroidaceae) and Spirochaetota (Breznakiellaceae) and reflect a specialization on cellodextrins, whereas their hemicellulolytic arsenal features activities on xylans and diverse heteropolymers. By contrast, the MAGs derived from flagellate-free HT possess a comprehensive arsenal of exo- and endoglucanases that resembles that of termite gut flagellates, underlining that Fibrobacterota and Spirochaetota occupy the cellulolytic niche that became vacant after the loss of the flagellates. Our analysis of the Fibrobacterota phylum reveal that all lineages possess a T9SS-like effectormechanism for the secretion of cellulases. Furthermore, we detected lignin-modifying enzymes in MAGs from HT assigned to the phyla Pseudomonadota (Burkholderiales, Pseudomonadales) and Actinomycetota (Actinomycetales, Mycobacteriales) located at the hindgut wall. The results of this study allow to refine our concept of symbiotic digestion of lignocellulose in termite guts, emphasizing the differential roles of specific bacterial lineages in both the flagellate-dependent and flagellate-independent breakdown of cellulose and hemicelluloses, and a so far unappreciated role of oxygen in the depolymerization of plant fiber and lignin in the microoxic periphery during gut passage in HT.