Chao Chen

Docosahexaenoic acid (DHA) is an essential ω-3 polyunsaturated fatty acid (PUFA) with high nutritional and pharmaceutical value. The marine protist Aurantiochytrium is a promising industrial DHA producer; however, its DHA biosynthesis via the PUFA synthase pathway co-produces ω-6 docosapentaenoic acid (DPA), limiting DHA purity. Here, we introduced an ω-3 desaturase from Phytophthora infestans (Pin-O3D) into Aurantiochytrium sp. SD116. Functional validation in an Escherichia coli system co-expressing the native PUFA synthase confirmed that Pin-O3D converts DPA to DHA, shifting the DHA/DPA ratio from 1:1 to 2:1. Pin-O3D was then integrated into the fatty acid synthase (FAS) locus, simultaneously attenuating FAS activity and enabling heterologous gene expression. The engineered strain ΔFAS-Pin-O3D exhibited significantly (p < 0.0001 in t-test) increased DHA content (55.2% of total fatty acids) and DHA/DPA ratio (5.91) in shake flasks, with no negative impact on biomass or lipid accumulation. Fed-batch fermentation confirmed the scalability of this strategy, achieving a >20% increase in DHA/DPA ratio. This study demonstrates that combining heterologous ω-3 desaturase expression with FAS attenuation is an effective approach for optimizing PUFA profiles in Aurantiochytrium.

Cellulosomes are sophisticated multi-enzyme complexes synthesized and secreted by anaerobic microorganisms, characterized by intricate structural components and highly organized modular assembly mechanisms. These complexes play a pivotal role in the efficient degradation of lignocellulosic biomass, significantly enhancing its bioconversion efficiency, and are thus regarded as invaluable enzymatic molecular machines. Cellulosomes are not only prevalent in anaerobic bacteria from soil and compost environments but are also integral to the digestive systems of herbivorous animals, primates and termites. The cellulosomes produced by digestive tract microbiota exhibit unique properties, providing novel enzymes and protein modules that are instrumental in biomass conversion and synthetic biology, thereby showcasing substantial application potential. Despite their promise, the isolation and cultivation of digestive tract microorganisms that produce cellulosomes present significant challenges. Additionally, the lack of comprehensive genetic and biochemical studies has impeded a thorough understanding of these cellulosomes, leaving them largely underexplored. This paper provides a comprehensive overview of the digestive tract cellulosome system, with a particular focus on the structural and functional attributes of cellulosomes in various animal digestive tracts. It also discusses the application prospects of digestive tract cellulosomes, highlighting their potential as a treasure in diverse fields.

In Clostridium thermocellum, there are nine RsgI factors responsible for sensing different types of substrates and regulating the transcription and expression of cellulosome genes. Within the signaling pathway of RsgI, the membrane protease RseP cleaves RsgI in its transmembrane helix, thus releasing the N-terminal fragment of RsgI from the membrane. This released RsgI N-terminal fragment is subsequently recognized and degraded by a cytoplasmic protease complex consisting of an AAA+ ATPase and ClpP protease. Previous research showed that the ClpXP complex, comprising ClpX and ClpP, is capable of recognizing and degrading the N-terminal fragment of RsgI6. However, due to the low conservation of the transmembrane helical region of RsgI, it remains unclear whether other RsgIs are similarly recognized and degraded by the same unfoldase. In this study, we employed in vitro protease assays to examine the recognition and degradation of the N-terminal fragment of each RsgI by various ClpP-unfoldase complexes. Results confirm that ClpXP is responsible for degrading the N-terminal fragments of all RsgI proteins in C. thermocellum, suggesting a degree of sequence promiscuity in substrate recognition by ClpXP. ClpXP can recognize multiple XAA sites in the transmembrane helix region of RsgI. Moreover, we unexpectedly discovered that the cytoplasmic domain influences the degradation of RsgI2-NF by ClpXP in our in vitro assay. This study provides new insights into understanding the complex regulatory mechanisms of cellulosome genes and the role of AAA+ proteases in C. thermocellum, thereby offering critical clues for unraveling the internal regulatory networks of bacteria.

Autoproteolysis has been discovered to play key roles in various biological processes, but functional autoproteolysis has been rarely reported for transmembrane signaling in prokaryotes. In this study, an autoproteolytic effect was discovered in the conserved periplasmic domain of anti-σ factor RsgIs from Clostridium thermocellum , which was found to transmit extracellular polysaccharide-sensing signals into cells for regulation of the cellulosome system, a polysaccharide-degrading multienzyme complex. Crystal and NMR structures of periplasmic domains from three RsgIs demonstrated that they are different from all known proteins that undergo autoproteolysis. The RsgI-based autocleavage site was located at a conserved Asn-Pro motif between the β1 and β2 strands in the periplasmic domain. This cleavage was demonstrated to be essential for subsequent regulated intramembrane proteolysis to activate the cognate SigI, in a manner similar to that of autoproteolysis-dependent activation of eukaryotic adhesion G protein–coupled receptors. These results indicate the presence of a unique prevalent type of autoproteolytic phenomenon in bacteria for signal transduction.

The alternative σ factor TcdR controls the synthesis of two major enterotoxins: TcdA and TcdB in Clostridioides difficile. Four potential TcdR-dependent promoters in the pathogenicity locus of C. difficile showed different activities. In this study, we constructed a heterologous system in Bacillus subtilis to investigate the molecular basis of TcdR-dependent promoter activity. The promoters of the two major enterotoxins showed strong TcdR-dependent activity, while the two putative TcdR-dependent promoters in the upstream region of the tcdR gene did not show detectable activity, suggesting that the autoregulation of TcdR may need other unknown factors involved. Mutation analysis indicated that the divergent -10 region is the key determinant for different activities of the TcdR-dependent promoters. Analysis of the TcdR model predicted by AlphaFold2 suggested that TcdR should be classified into group 4, i.e., extracytoplasmic function, σ⁷⁰ factors. The results of this study provide the molecular basis of the TcdR-dependent promoter recognition for toxin production. This study also suggests the feasibility of the heterologous system in analyzing σ factor functions and possibly in drug development targeting these factors.

Many important proteins undergo pH-dependent conformational changes resulting in “on-off” switches for protein function, which are essential for regulation of life processes and have wide application potential. Here, we report a pair of cellulosomal assembly modules, comprising a cohesin and a dockerin from Clostridium acetobutylicum, which interact together following a unique pH-dependent switch between two functional sites rather than on-off states. The two cohesin-binding sites on the dockerin are switched from one to the other at pH 4.8 and 7.5 with a 180° rotation of the bound dockerin. Combined analysis by nuclear magnetic resonance spectroscopy, crystal structure determination, mutagenesis, and isothermal titration calorimetry elucidates the chemical and structural mechanism of the pH-dependent switching of the binding sites. The pH-dependent dual-binding-site switch not only represents an elegant example of biological regulation but also provides a new approach for developing pH-dependent protein devices and biomaterials beyond an on-off switch for biotechnological applications.

Bacterial insertion sequences (ISs) are ubiquitous mobile genetic elements that play important roles in genome plasticity, cell adaptability, and function evolution. ISs of various families and subgroups contain significantly diverse molecular features and functional mechanisms that are not fully understood. IS1447 is a member of the widespread IS3 family and was previously detected to have transposing activity in a typical thermophilic and cellulolytic microorganism Clostridium thermocellum. Phylogenetic analysis showed that IS1447-like elements are widely distributed in Firmicutes and possess unique features in the IS3 family. Therefore, IS1447 may represent a novel subgroup of the IS3 family. Unlike other well-known IS3 subgroups performing programmed − 1 translational frameshifting for the expression of the transposase, IS1447 exhibits transcriptional slippage in both the + 1 and − 1 directions, each with a frequency of ~ 16%, and only + 1 slippage results in full-length and functional transposase. The slippage-prone region of IS1447 contains a run of nine A nucleotides following a stem-loop structure in mRNA, but mutagenesis analysis indicated that seven of them are sufficient for the observed slippage. Western blot analysis indicated that IS1447 produces three types of transposases with alternative initiations. Furthermore, the IS1447-subgroup elements are abundant in the genomes of several cellulolytic bacteria. Our result indicated that IS1447 represents a new Firmicutes-enriched subgroup of the IS3 family. The characterization of the novel IS3-family member will enrich our understanding of the transposition behavior of IS elements and may provide insight into developing IS-based mutagenesis tools for thermophiles.