An Outer Membrane Protein Involved in the Uptake of Glucose Is Essential for Cytophaga hutchinsonii Cellulose Utilization

Small GTPase Rab7 is involved in stress adaptation to carbon starvation to ensure the induced cellulase biosynthesis in Trichoderma reesei

BACKGROUND

The saprophytic filamentous fungus Trichoderma reesei represents one of the most prolific cellulase producers. The bulk production of lignocellulolytic enzymes by T. reesei not only relies on the efficient transcription of cellulase genes but also their efficient secretion after being translated. However, little attention has been paid to the functional roles of the involved secretory pathway in the high-level production of cellulases in T. reesei. Rab GTPases are key regulators in coordinating various vesicle trafficking associated with the eukaryotic secretory pathway. Specifically, Rab7 is a representative GTPase regulating the transition of the early endosome to the late endosome followed by its fusion to the vacuole as well as homotypic vacuole fusion. Although crosstalk between the endosomal/vacuolar pathway and the secretion pathway has been reported, the functional role of Rab7 in cellulase production in T. reesei remains unknown.

RESULTS

A TrRab7 was identified and characterized in T. reesei. TrRab7 was shown to play important roles in T. reesei vegetative growth and vacuole morphology. Whereas knock-down of Trrab7 significantly compromised the induced production of T. reesei cellulases, overexpression of the key transcriptional activator, Xyr1, restored the production of cellulases in the Trrab7 knock-down strain (Ptcu-rab7KD) on glucose, indicating that the observed defective cellulase biosynthesis results from the compromised cellulase gene transcription. Down-regulation of Trrab7 was also found to make T. reesei more sensitive to various stresses including carbon starvation. Interestingly, overexpression of Snf1, a serine/threonine protein kinase known as an energetic sensor, partially restored the cellulase production of Ptcu-rab7KD on Avicel, implicating that TrRab7 is involved in an energetic adaptation to carbon starvation which contributes to the successful cellulase gene expression when T. reesei is transferred from glucose to cellulose.

CONCLUSIONS

TrRab7 was shown to play important roles in T. reesei development and a stress response to carbon starvation resulting from nutrient shift. This adaptation may allow T. reesei to successfully initiate the inducing process leading to efficient cellulase production. The present study provides useful insights into the functional involvement of the endosomal/vacuolar pathway in T. reesei development and hydrolytic enzyme production.

Glycosyltransferase-Related Protein GtrA Is Essential for Localization of Type IX Secretion System Cargo Protein Cellulase Cel9A and Affects Cellulose Degradation in Cytophaga hutchinsonii

The Gram-negative bacterium Cytophaga hutchinsonii digests cellulose through a novel cellulose degradation mechanism. It possesses the lately characterized type IX secretion system (T9SS). We recently discovered that N-glycosylation of the C-terminal domain (CTD) of a hypothetical T9SS substrate protein in the periplasmic space of C. hutchinsonii affects protein secretion and localization. In this study, green fluorescent protein (GFP)-CTD_Cel9A recombinant protein was found with increased molecular weight in the periplasm of C. hutchinsonii. Site-directed mutagenesis studies on the CTD of cellulase Cel9A demonstrated that asparagine residue 900 in the D-X-N-X-S motif is important for the processing of the recombinant protein. We found that the glycosyltransferase-related protein GtrA (CHU_0012) located in the cytoplasm of C. hutchinsonii is essential for outer membrane localization of the recombinant protein. The deletion of gtrA decreased the abundance of the outer membrane proteins and affected cellulose degradation by C. hutchinsonii. This study provided a link between the glycosylation system and cellulose degradation in C. hutchinsonii. IMPORTANCE N-Glycosylation systems are generally limited to some pathogenic bacteria in prokaryotes. The disruption of the N-glycosylation pathway is related to adherence, invasion, colonization, and other phenotypic characteristics. We recently found that the cellulolytic bacterium Cytophaga hutchinsonii also has an N-glycosylation system. The cellulose degradation mechanism of C. hutchinsonii is novel and mysterious; cellulases and other proteins on the cell surface are involved in utilizing cellulose. In this study, we identified an asparagine residue in the C-terminal domain of cellulase Cel9A that is necessary for the processing of the T9SS cargo protein. Moreover, the glycosyltransferase-related protein GtrA is essential for the localization of the GFP-CTD_Cel9A recombinant protein. Deletion of gtrA affected cellulose degradation and the abundance of outer membrane proteins. This study enriched the understanding of the N-glycosylation system in C. hutchinsonii and provided a link between N-glycosylation and cellulose degradation, which also expanded the role of the N-glycosylation system in bacteria.

A T9SS Substrate Involved in Crystalline Cellulose Degradation by Affecting Crucial Cellulose Binding Proteins in Cytophaga hutchinsonii

Cytophaga hutchinsonii is an abundant soil cellulolytic bacterium that uses a unique cellulose degradation mechanism different from those that involve free cellulases or cellulosomes. Though several proteins were identified to be important for cellulose degradation, the mechanism used by C. hutchinsonii to digest crystalline cellulose remains a mystery. In this study, chu_0922 was identified by insertional mutation and gene deletion as an important gene locus indispensable for crystalline cellulose utilization. Deletion of chu_0922 resulted in defect in crystalline cellulose utilization. The Δ0922 mutant completely lost the ability to grow on crystalline cellulose even with extended incubation, and selectively utilized the amorphous region of cellulose leading to the increased crystallinity. As a protein secreted by the type Ⅸ secretion system (T9SS), CHU_0922 was found to be located on the outer membrane, and the outer membrane localization of CHU_0922 relied on the T9SS. Comparative analysis of the outer membrane proteins revealed that the abundance of several cellulose binding proteins, including CHU_1276, CHU_1277, and CHU_1279, was reduced in the Δ0922 mutant. Further study showed that CHU_0922 is crucial for the full expression of the gene cluster containing chu_1276, chu_1277, chu_1278, chu_1279, and chu_1280 (cel9C), which is essential for cellulose utilization. Moreover, CHU_0922 is required for the cell surface localization of CHU_3220, a cellulose binding protein that is essential for crystalline cellulose utilization. Our study provides insights into the complex system that C. hutchinsonii uses to degrade crystalline cellulose. IMPORTANCE The widespread aerobic cellulolytic bacterium Cytophaga hutchinsonii, belonging to the phylum Bacteroidetes, utilizes a novel mechanism to degrade crystalline cellulose. No genes encoding proteins specialized in loosening or disruption the crystalline structure of cellulose were identified in the genome of C. hutchinsonii, except for chu_3220 and chu_1557. The crystalline cellulose degradation mechanism remains enigmatic. This study identified a new gene locus, chu_0922, encoding a typical T9SS substrate that is essential for crystalline cellulose degradation. Notably, CHU_0922 is crucial for the normal transcription of chu_1276, chu_1277, chu_1278, chu_1279, and chu_1280 (cel9C), which play important roles in the degradation of cellulose. Moreover, CHU_0922 participates in the cell surface localization of CHU_3220. These results demonstrated that CHU_0922 plays a key role in the crystalline cellulose degradation network. Our study will promote the uncovering of the novel cellulose utilization mechanism of C. hutchinsonii.

Changes in bacterial diversity of activated sludge exposed to titanium dioxide nanoparticles

The rapid growth of the use of nanomaterials in different modern industrial branches makes the study of the impact of nanoparticles on the human health and environment an urgent matter. For instance, it has been reported that titanium dioxide nanoparticles (TiO₂ NPs) can be found in wastewater treatment plants. Previous studies have found contrasting effects of these nanoparticles over the activated sludge process, including negative effects on the oxygen uptake. The non-utilization of oxygen reflects that aerobic bacteria were inhibited or decayed. The aim of this work was to study how TiO₂ NPs affect the bacterial diversity and metabolic processes on an activated sludge. First, respirometry assays of 8 h were carried out at different concentrations of TiO₂ NPs (0.5-2.0 mg/mL) to measure the oxygen uptake by the activated sludge. The bacterial diversity of these assays was determined by sequencing the amplified V3-V4 region of the 16S rRNA gene using Illumina MiSeq. According to the respirometry assays, the aerobic processes were inhibited in a range from 18.5 ± 4.8% to 37.5 ± 2.0% for concentrations of 0.5-2.0 mg/mL TiO₂ NPs. The oxygen uptake rate was affected mainly after 4.5 h for concentrations higher than 1.0 mg/mL of these nanoparticles. Results indicated that, in the presence of TiO₂ NPs, the bacterial community of activated sludge was altered mainly in the genera related to nitrogen removal (nitrogen assimilation, nitrification and denitrification). The metabolic pathways prediction suggested that genes related to biofilm formation were more sensitive than genes directly related to nitrification-denitrification and N-assimilation processes. These results indicated that TiO₂ NPs might modify the bacteria diversity in the activated sludge according to their concentration and time of exposition, which in turn impact in the performance of the wastewater treatment processes.

Cytophaga hutchinsonii SprA and SprT Are Essential Components of the Type IX Secretion System Required for Ca2+ Acquisition, Cellulose Degradation, and Cell Motility

The type IX secretion system (T9SS) is a novel protein secretion system, which is found in and confined to the phylum Bacteroidetes. T9SS is involved in the secretion of virulence factors, cell surface adhesins, and complex biopolymer degrading enzymes to the cell surface or extracellular medium. Cytophaga hutchinsonii is a widely distributed bacterium, which is able to efficiently digest cellulose and rapidly glide along the solid surfaces. C. hutchinsonii has a full set of orthologs of T9SS components. However, the functions of most homologous proteins have not been verified. In C. hutchinsonii, CHU_0029 and CHU_2709 are similar in sequence to Flavobacterium johnsoniae T9SS components SprA and SprT, respectively. In this study, the single deletion mutants of chu_0029 (sprA) and chu_2709 (sprT) were obtained using a complex medium with the addition of Ca²⁺ and Mg²⁺. Single deletion of sprA or sprT resulted in defects in cellulose utilization and gliding motility. Moreover, the ΔsprA and ΔsprT mutants showed growth defects in Ca²⁺- and Mg²⁺-deficient media. The results of ICP-MS test showed that both the whole cell and intracellular concentrations of Ca²⁺ were dramatically reduced in the ΔsprA and ΔsprT mutants, indicating that SprA and SprT are both important for the assimilation of trace amount of Ca²⁺. While the assimilation of Mg²⁺ was not obviously influenced in the ΔsprA and ΔsprT mutants. Through proteomics analysis of the cell surface proteins of the wild type and mutants, we found that the ΔsprA and ΔsprT mutants were defective in secretion of the majority of T9SS substrates. Together, these results indicate that SprA and SprT are both essential components of C. hutchinsonii T9SS, which is required for protein secretion, Ca²⁺ acquisition, cellulose degradation, and gliding motility in C. hutchinsonii. Our study shed more light on the functions of SprA and SprT in T9SS, and further proved the link between the T9SS and Ca²⁺ uptake system.

Cytophaga hutchinsonii gldN, Encoding a Core Component of the Type IX Secretion System, Is Essential for Ion Assimilation, Cellulose Degradation, and Cell Motility

The type IX secretion system (T9SS), which is involved in pathogenicity, motility, and utilization of complex biopolymers, is a novel protein secretion system confined to the phylum Bacteroidetes Cytophaga hutchinsonii, a common cellulolytic soil bacterium belonging to the phylum Bacteroidetes, can rapidly digest crystalline cellulose using a novel strategy. In this study, the deletion mutant of chu_0174 (gldN) was obtained using PY6 medium supplemented with Stanier salts. GldN was verified to be a core component of C. hutchinsonii T9SS, and is indispensable for cellulose degradation, motility, and secretion of C-terminal domain (CTD) proteins. Notably, the ΔgldN mutant showed significant growth defects in Ca²⁺- and Mg²⁺-deficient media. These growth defects could be relieved by the addition of Ca²⁺ or Mg²⁺ The intracellular concentrations of Ca²⁺ and Mg²⁺ were markedly reduced in ΔgldN These results demonstrated that GldN is essential for the acquisition of trace amounts of Ca²⁺ and Mg²⁺, especially for Ca²⁺ Moreover, an outer membrane efflux protein, CHU_2807, which was decreased in abundance on the outer membrane of ΔgldN, is essential for normal growth in PY6 medium. The reduced intracellular accumulation of Ca²⁺ and Mg²⁺ in the Δ2807 mutant indicated that CHU_2807 is involved in the uptake of trace amounts of Ca²⁺ and Mg²⁺ This study provides insights into the role of T9SS in metal ion assimilation in C. hutchinsonii IMPORTANCE The widespread Gram-negative bacterium Cytophaga hutchinsonii uses a novel but poorly understood strategy to utilize crystalline cellulose. Recent studies showed that a T9SS exists in C. hutchinsonii and is involved in cellulose degradation and motility. However, the main components of the C. hutchinsonii T9SS and their functions are still unclear. Our study characterized the function of GldN, which is a core component of the T9SS. GldN was proved to play vital roles in cellulose degradation and cell motility. Notably, GldN is essential for the acquisition of Ca²⁺ and Mg²⁺ ions under Ca²⁺- and Mg²⁺-deficient conditions, revealing a link between the T9SS and the metal ion transport system. The outer membrane abundance of CHU_2807, which is essential for Ca²⁺ and Mg²⁺ uptake in PY6 medium, was affected by the deletion of GldN. This study demonstrated that the C. hutchinsonii T9SS has extensive functions, including cellulose degradation, motility, and metal ion assimilation, and contributes to further understanding of the function of the T9SS in the phylum Bacteroidetes.

A Disulfide Oxidoreductase (CHU_1165) Is Essential for Cellulose Degradation by Affecting Outer Membrane Proteins in Cytophaga hutchinsonii

Cytophaga hutchinsonii cells can bind to the surface of insoluble cellulose and degrade it by utilizing a novel cell contact-dependent mechanism, in which the outer membrane proteins may play important roles. In this study, the deletion of a gene locus, chu_1165, which encodes a hypothetical protein with 32% identity with TlpB, a disulfide oxidoreductase in Flavobacterium psychrophilum, caused a complete cellulolytic defect in C. hutchinsonii Further study showed that cells of the Δ1165 strain could not bind to cellulose, and the levels of many outer membrane proteins that can bind to cellulose were significantly decreased. The N-terminal region of CHU_1165 is anchored to the cytoplasmic membrane with five predicted transmembrane helices, and the C-terminal region is predicted to stretch to the periplasm and has a similar thioredoxin (Trx) fold containing a Cys-X-X-Cys motif that is conserved in disulfide oxidoreductases. Recombinant CHU_1165_His containing the Cys-X-X-Cys motif was able to reduce the disulfide bonds of insulin in vitro Site-directed mutation showed that the cysteines in the Cys-X-X-Cys motif and at residues 106 and 108 were indispensable for the function of CHU_1165. Western blotting showed that CHU_1165 was in an oxidized state in vivo, suggesting that it may act as an oxidase to catalyze disulfide bond formation. However, many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of the cysteine in these proteins did not affect cellulose degradation, indicating that CHU_1165 may have an indirect or pleiotropic effect on the function of these outer membrane proteins.IMPORTANCE Cytophaga hutchinsonii can rapidly digest cellulose in a contact-dependent manner, in which the outer membrane proteins may play important roles. In this study, a hypothetical protein, CHU_1165, characterized as a disulfide oxidoreductase, is essential for cellulose degradation by affecting the cellulose binding ability of many outer membrane proteins in C. hutchinsonii Disulfide oxidoreductases are involved in disulfide bond formation. However, our studies show that many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of cysteine did not affect their function, indicating that CHU_1165 did not facilitate the formation of a disulfide bond in these proteins. It may have an indirect or pleiotropic effect on the function of these outer membrane proteins. Our study provides an orientation for exploring the proteins that assist in the appropriate conformation of many outer membrane proteins essential for cellulose degradation, which is important for exploring the novel mechanism of cellulose degradation in C. hutchinsonii.

Identification of a cell-surface protein involved in glucose assimilation and disruption of the crystalline region of cellulose by Cytophaga hutchinsonii

The crystalline region of cellulose is the main barrier to the utilization of crystalline cellulose. Cytophaga hutchinsonii actively digests the crystalline region of cellulose by an unknown mechanism. Transposon mutagenesis was done to identify a novel gene locus chu_1557, which is required for efficient disruption of the crystalline region of cellulose, and the absence of CHU_1557 resulted in decreased glucose assimilation efficiency. The defect of the mutant in the disruption of the crystalline region of cellulose was partially retained by additional glucose or pre-culturing the mutant in a low glucose concentration medium which could improve its glucose absorption efficiency. These results suggested that extracellular glucose has important roles in the disruption of crystalline cellulose by C. hutchinsonii. Further study showed that the expression of an outer membrane protein CHU_3732 was downregulated by the absence of CHU_1557 in a low glucose concentration medium. CHU_3732 was involved in uptake of glucose and its expression was induced by a low concentration of glucose. CHU_3732 was predicted to be a porin, so we inferred that it may work as a glucose transport channel in the outer membrane. Based on these results, we deduced that CHU_1557 played a role in the process of glucose assimilation and its disruption affected the expression of other proteins related to glucose transportation such as CHU_3732, and then affected the cell growth in a low glucose concentration medium and disruption of the crystalline region of cellulose.

An Extracytoplasmic Function Sigma Factor Controls Cellulose Utilization by Regulating the Expression of an Outer Membrane Protein in Cytophaga hutchinsonii

The common soil cellulolytic bacterium known as Cytophaga hutchinsonii makes use of a unique but poorly understood strategy in order to utilize cellulose. While several genes have been identified as being an active part of the utilization of cellulose, the mechanism(s) by which C. hutchinsonii both (i) senses its environment and (ii) regulates the expression of those genes are not as yet known. In this study, we identified and characterized the gene CHU_3097 encoding an extracytoplasmic function (ECF) σ factor (σ^cel1), the disruption of which compromised C. hutchinsonii cellulose assimilation to a large degree. The σ^cel1 and its putative partner anti-σ^cel1, encoded by the CHU_3096 gene found immediately downstream from CHU_3097, copurified in vitro The σ^cel1 was discovered to be associated with inner membrane when cells were cultured on glucose and yet was partially released from the membrane in response to cellulose. This release was found to occur on glucose when the anti-σ^cel1 was absent. Transcriptome analyses found a σ^cel1-regulated, cellulose-responsive gene regulon, within which an outer membrane protein encoding the gene CHU_1276, essential for cellulose utilization, was discovered to be significantly downregulated by CHU_3097 disruption. The expression of CHU_1276 almost fully restored cellulose utilization to the CHU_3097 mutant, demonstrating that CHU_1276 represents a critical regulatory target of σ^cel1 In this way, our study provided insights into the role of an ECF σ factor in coordinating the cellulolytic response of C. hutchinsonii IMPORTANCE The common cellulolytic bacterium Cytophaga hutchinsonii uses a unique but poorly understood strategy in order to make use of cellulose. Throughout the process of cellulosic biomass breakdown, outer membrane proteins are thought to play key roles; this is evidenced by CHU_1276, which is required for the utilization of cellulose. However, the regulatory mechanism of its expression is not yet known. We found and characterized an extracytoplasmic function σ factor that is involved in coordinating the cellulolytic response of C. hutchinsonii by directly regulating the expression of CHU_1276 This study makes a contribution to our understanding of the regulatory mechanism used by C. hut chinsonii in order to adjust its genetic programs and so deal with novel environmental cues.

Effects of the histone-like protein HU on cellulose degradation and biofilm formation of Cytophaga hutchinsonii

Cytophaga hutchinsonii, belonging to Bacteroidetes, is speculated to use a novel cell-contact mode to digest cellulose. In this study, we identified a histone-like protein HU, CHU_2750, in C. hutchinsonii, whose transcription could be induced by crystalline but not amorphous cellulose. We constructed a CHU_2750-deleted mutant and expressed CHU_2750 in Escherichia coli to study the gene's functions. Our results showed that although the deletion of CHU_2750 was not lethal to C. hutchinsonii, the mutant displayed an abnormal filamentous morphology, loose nucleoid, and obvious defects in the degradation of crystalline cellulose and cell motility. Further study indicated that the mutant displayed significantly decreased cell surface and intracellular endoglucanase activities but with β-glucosidase activities similar to the wild-type strain. Analyses by real-time quantitative PCR revealed that the transcription levels of many genes involved in cellulose degradation and/or cell motility were significantly downregulated in the mutant. In addition, we found that CHU_2750 was important for biofilm formation of C. hutchinsonii. The main extracellular components of the biofilm were analyzed, and the results showed that the mutant yielded significantly less exopolysaccharide but more extracellular DNA and protein than the wild-type strain. Collectively, our findings demonstrated that CHU_2750 is important for cellulose degradation, cell motility, and biofilm formation of C. hutchinsonii by modulating transcription of certain related genes, and it is the first identified transcriptional regulator in these processes of C. hutchinsonii. Our study shed more light on the mechanisms of cellulose degradation, cell motility, and biofilm formation by C. hutchinsonii.

Deletion of a Gene Encoding a Putative Peptidoglycan-Associated Lipoprotein Prevents Degradation of the Crystalline Region of Cellulose in Cytophaga hutchinsonii

Cytophaga hutchinsonii is a gliding Gram-negative bacterium in the phylum Bacteroidetes with the capability to digest crystalline cellulose rapidly, but the mechanism is unclear. In this study, deletion of chu_0125, encoding a homolog of the peptidoglycan-associated lipoprotein (Pal), was determined to prevent degradation of the crystalline region of cellulose. We found that the chu_0125 deletion mutant grew normally in regenerated amorphous cellulose medium but displayed defective growth in crystalline cellulose medium and increased the degree of crystallinity of Avicel. The endoglucanase and β-glucosidase activities on the cell surface were reduced by 60 and 30% without chu_0125, respectively. Moreover, compared with the wild type, the chu_0125 deletion mutant was found to be more sensitive to some harmful compounds and to release sixfold more outer membrane vesicles (OMVs) whose protein varieties were dramatically increased. These results indicated that CHU_0125 played a critical role in maintaining the integrity of the outer membrane. Further study showed that the amounts of some outer membrane proteins were remarkably decreased in the chu_0125 deletion mutant. Western blotting revealed that CHU_3220, the only reported outer membrane protein that was necessary and specialized for degradation of the crystalline region of cellulose, was largely leaked from the outer membrane and packaged into OMVs. We concluded that the deletion of chu_0125 affected the integrity of outer membrane and thus influenced the localization of some outer membrane proteins including CHU_3220. This might be the reason why deletion of chu_0125 prevented degradation of the crystalline region of cellulose.

Transcriptome and Zymogram Analyses Reveal a Cellobiose-Dose Related Reciprocal Regulatory Effect on Cellulase Synthesis in Cellulosilyticum ruminicola H1

The rumen bacterium Cellulosilyticum ruminicola H1 efficiently hydrolyzes cellulose. To gain insights into the regulatory mechanisms of cellulase synthesis, comparative transcriptome analysis was conducted for cultures grown on 2% filter paper, 0.5 and 0.05% cellobiose, and 0.5% birchwood xylan. It was found that cellulose induced a majority of (hemi)cellulases, including 33 cellulases and a cellulosomal scaffoldin (1.3- to 22.7-fold); seven endoxylanases, two mannanases, and two pectatelyases (2- to 16-fold); and pyruvate formate-lyase (PFL, 1.5- to 7-fold). Noticeably, 3- and 2.5-fold increased transcription of a cellobiohydrolase and the cellulosomal scaffoldin precursor were detected in 0.05% than in 0.5% cellobiose. Consistently, 9- and 4-fold higher specific cellobiohydrolase activities were detected in the filter paper and 0.05% cellobiose culture. SDS- and native-PAGE zymograms of cellulose-enriched proteins from the filter paper culture displayed cellulase activities, and cellulolytic “complexes” were enriched from the filter paper- and 0.05% cellobiose-cultures, but not from the 0.5% cellobiose culture. LC-MS/MS identified the cellulosomal scaffoldin precursor in the “complexes” in addition to cellulase, hemicellulase, and PFL proteins. The addition of 0.5% cellobiose, but not 0.05% cellobiose remarkably inhibited strain H1 to degrade filter paper. Therefore, this work reveals a cellobiose-dose related regulatory mechanism of cellulase synthesis by lower for induction and higher for repression, which has extended our understanding of the regulation of microbial cellulase synthesis.

The unusual cellulose utilization system of the aerobic soil bacterium Cytophaga hutchinsonii

Cellulolytic microorganisms play important roles in global carbon cycling and have evolved diverse strategies to digest cellulose. Some are 'generous,' releasing soluble sugars from cellulose extracellularly to feed both themselves and their neighbors. The gliding soil bacterium Cytophaga hutchinsonii exhibits a more 'selfish' strategy. It digests crystalline cellulose using cell-associated cellulases and releases little soluble sugar outside of the cell. The mechanism of C. hutchinsonii cellulose utilization is still poorly understood. In this review, we discuss novel aspects of the C. hutchinsonii cellulolytic system. Recently developed genetic manipulation tools allowed the identification of proteins involved in C. hutchinsonii cellulose utilization. These include periplasmic and cell-surface endoglucanases and novel cellulose-binding proteins. The recently discovered type IX secretion system is needed for cellulose utilization and appears to deliver some of the cellulolytic enzymes and other proteins to the cell surface. The requirement for periplasmic endoglucanases for cellulose utilization is unusual and suggests that cello-oligomers must be imported across the outer membrane before being further digested. Cellobiohydrolases or other predicted processive cellulases that play important roles in many other cellulolytic bacteria appear to be absent in C. hutchinsonii. Cells of C. hutchinsonii attach to and glide along cellulose fibers, which may allow them to find sites most amenable to attack. A model of C. hutchinsonii cellulose utilization summarizing recent progress is proposed.

A Putative Type II Secretion System Is Involved in Cellulose Utilization in Cytophaga hutchisonii

Cytophaga hutchinsonii is a gliding cellulolytic bacterium that degrades cellulose in a substrate contact-dependent manner. Specific proteins are speculated to be translocated to its extracellular milieu or outer membrane surface to participate in adhesion to cellulose and further digestion. In this study, we show that three orthologous genes encoding the major components (T2S-D, -F, and -G) of type II secretion system (T2SS) are involved in cellulose degradation but not in cell motility. The individual disruption of the three t2s genes results in a significantly retarded growth on cellobiose, regenerated amorphous cellulose, and Avicel cellulose. Enzymatic analyses demonstrate that, whereas the endoglucanase activity of the t2s mutant cells is increased, the β-glucosidase activity is remarkably reduced compared to that of WT cells. Further analyses reveal that the t2s mutant cells not only exhibit a different profile of cellulose-bound outer membrane proteins from that of wild-type cells, but also display a significant decrease in their capability to adhere to cellulose. These results indicate that a functional link exits between the putative T2SS and cellulose utilization in C. hutchinsonii, and thus provide a conceptual framework to understand the unique strategy deployed by C. hutchinsonii to assimilate cellulose.

Identification and Characterization of a Large Protein Essential for Degradation of the Crystalline Region of Cellulose by Cytophaga hutchinsonii

Cytophaga hutchinsonii is a Gram-negative bacterium that can efficiently degrade crystalline cellulose by a unique mechanism different from the free cellulase or cellulosome strategy. In this study, chu_3220, encoding the hypothetical protein CHU_3220 (205 kDa), was identified by insertional mutation and gene deletion as the first gene essential for degradation of the crystalline region but not the amorphous region of cellulose by C. hutchinsonii A chu_3220 deletion mutant was defective in the degradation of crystalline cellulose and increased the degree of crystallinity of Avicel PH101 but could still degrade amorphous cellulose completely. CHU_3220 was found to be located on the outer surface of the outer membrane and could bind to cellulose. It contains 15 PbH1 domains and a C-terminal domain (CHU_C) that was proved to be critical for the localization of CHU_3220 on the cell surface and the function of CHU_3220 in crystalline cellulose degradation. Moreover, the degradation of crystalline cellulose was intact-cell dependent and inhibited by NaN₃ Further study showed that chu_3220 was induced by cellulose and that the endoglucanase activity on the cell surface was significantly reduced without chu_3220 Real-time PCR revealed that the transcription of most genes encoding endoglucanases located on the cell surface was decreased in the chu_3220 deletion mutant, indicating that chu_3220 might also play a role in the regulation of the expression of some endoglucanases.

IMPORTANCE

Cytophaga hutchinsonii could efficiently degrade crystalline cellulose with a unique mechanism without cellulosomes and free cellulases. It lacks proteins that are thought to play important roles in disruption of the crystalline region of cellulose, including exoglucanases, lytic polysaccharide monooxygenases, expansins, expansin-like proteins, or swollenins, and most of its endoglucanases lack carbohydrate binding modules. The mechanism of the degradation of crystalline cellulose is still unknown. In this study, chu_3220 was identified as the first gene essential for the degradation of the crystalline region but not the amorphous region of cellulose. CHU_3220 is a high-molecular-weight protein located on the outer surface of the outer membrane and could bind to cellulose. We proposed that CHU_3220 might be an essential component of a protein complex on the cell surface in charge of the decrystallization of crystalline cellulose. The degradation of crystalline cellulose by C. hutchinsonii was not only dependent on intact cells but also required the energy supplied by the cells. This was obviously different from other known cellulose depolymerization system. Our study has shed more light on the novel strategy of crystalline cellulose degradation by C. hutchinsonii.

Periplasmic Cytophaga hutchinsonii Endoglucanases Are Required for Use of Crystalline Cellulose as the Sole Source of Carbon and Energy

The soil bacterium Cytophaga hutchinsonii actively digests crystalline cellulose by a poorly understood mechanism. Genome analyses identified nine genes predicted to encode endoglucanases with roles in this process. No predicted cellobiohydrolases, which are usually involved in the utilization of crystalline cellulose, were identified. Chromosomal deletions were performed in eight of the endoglucanase-encoding genes: cel5A, cel5B, cel5C, cel9A, cel9B, cel9C, cel9E, and cel9F Each mutant retained the ability to digest crystalline cellulose, although the deletion of cel9C caused a modest decrease in cellulose utilization. Strains with multiple deletions were constructed to identify the critical cellulases. Cells of a mutant lacking both cel5B and cel9C were completely deficient in growth on cellulose. Cell fractionation and biochemical analyses indicate that Cel5B and Cel9C are periplasmic nonprocessive endoglucanases. The requirement of periplasmic endoglucanases for cellulose utilization suggests that cellodextrins are transported across the outer membrane during this process. Bioinformatic analyses predict that Cel5A, Cel9A, Cel9B, Cel9D, and Cel9E are secreted across the outer membrane by the type IX secretion system, which has been linked to cellulose utilization. These secreted endoglucanases may perform the initial digestion within amorphous regions on the cellulose fibers, releasing oligomers that are transported into the periplasm for further digestion by Cel5B and Cel9C. The results suggest that both cell surface and periplasmic endoglucanases are required for the growth of C. hutchinsonii on cellulose and that novel cell surface proteins may solubilize and transport cellodextrins across the outer membrane.

IMPORTANCE

The bacterium Cytophaga hutchinsonii digests crystalline cellulose by an unknown mechanism. It lacks processive cellobiohydrolases that are often involved in cellulose digestion. Critical cellulolytic enzymes were identified by genetic analyses. Intracellular (periplasmic) nonprocessive endoglucanases performed an important role in cellulose utilization. The results suggest a model involving partial digestion at the cell surface, solubilization and uptake of cellodextrins across the outer membrane by an unknown mechanism, and further digestion within the periplasm. The ability to sequester cellodextrins and digest them intracellularly may limit losses of soluble cellobiose to other organisms. C. hutchinsonii uses an unusual approach to digest cellulose and is a potential source of novel proteins to increase the efficiency of conversion of cellulose into soluble sugars and biofuels.