Bactofencin YH, a novel bacteriocin with high inhibitory activity against clinical Streptococcus species.

Strain Lactiplantibacillus plantarum D1 with bacteriocin producing ability was found in the intestine of Gambusia affinis. The bacteriocin was found to have high inhibitory activity against multiple Streptococcus species and several other Gram-positive and Gram-negative bacteria. Bacteriocin was purified from culture supernatant by ion-exchange chromatography, Sep-Pak C18 cartridge, and reverse-phase high-performance liquid chromatography (RP-HPLC). Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectral analysis determined that purified bacteriocin has a molecular mass of 2,731 Da. A partial N-terminal sequence KRKKHKXQIYNNGM was obtained from the Edman analysis. The N-terminal sequence was employed to search against a translation of the draft genome of strain D1. The translated full amino acid sequence of the mature peptide is as follows: NH2- KRKKHKCQIYNNGMPTGQYRWC, which has a molecular weight of 2738 Da. A BLAST search revealed that this bacteriocin was most similar to bactofencin A but differed from it with three amino acid residues. No identical peptide or protein has been previously reported, and this peptide, termed bactofencin YH, was therefore considered to be a new bacteriocin produced by Lactiplantibacillus plantarum D1.

Silica-associated proteins from hexactinellid sponges support an alternative evolutionary scenario for biomineralization in Porifera.

Metazoans use silicon traces but rarely develop extensive silica skeletons, except for the early-diverging lineage of sponges. The mechanisms underlying metazoan silicification remain incompletely understood, despite significant biotechnological and evolutionary implications. Here, the characterization of two proteins identified from hexactinellid sponge silica, hexaxilin and perisilin, supports that the three classes of siliceous sponges (Hexactinellida, Demospongiae, and Homoscleromorpha) use independent protein machineries to build their skeletons, which become non-homologous structures. Hexaxilin forms the axial filament to intracellularly pattern the main symmetry of the skeletal parts, while perisilin appears to operate in their thickening, guiding extracellular deposition of peripheral silica, as does glassin, a previously characterized hexactinellid silicifying protein. Distant hexaxilin homologs occur in some bilaterians with siliceous parts, suggesting putative conserved silicifying activity along metazoan evolution. The findings also support that ancestral Porifera were non-skeletonized, acquiring silica skeletons only after diverging into major classes, what reconciles molecular-clock dating and the fossil record.

Acetylation of amines and alcohols catalyzed by acetylcholinesterase from Pseudomonas aeruginosa PAO1.

Acetylcholinesterase (AChE) from Pseudomonas aeruginosa PAO1 has a catalytic Ser residue in its active site. In this study, we examined the aminolysis and alcoholysis reactions of AChE that occurred alongside its hydrolysis reaction. The recombinant AChE recognized ethyl acetate as a substrate. Therefore, we evaluated acetylation of the amine and hydroxyl group by AChE, using acetylcholine and ethyl acetate as the acetyl donor. AChE recognized diaminoalkanes with 4- to 12-carbon chains and aminoalcohols with 4- to 8-carbon chains as acetyl acceptors, resulting in their acetylated products. In the acetylation of 1,6-diaminohexane, AChE preferentially used ethyl acetate as the acetyl donor above pH 8.0 and the efficiency increased with increasing pH. In contrast, the acetylation of 6-amino-1-hexanol was efficient with acetylcholine as the acetyl donor in the pH range of 4-10. In addition, acetylated 6-amino-1-hexanol was decomposed by AChE. The kinetic study indicated that the acetyl donor and acceptor are competitively recognized by AChE as substrates.

Production and characterization of cyanocobalamin-enriched tomato (Solanum lycopersicum) fruits grown using hydroponics.

BACKGROUND: Vitamin B12 is an essential vitamin that is absent in plant-derived foods such as fruits and vegetables. This can result in an increased risk of developing vitamin B12 deficiency in strict vegetarians (vegans). There are several studies that have aimed to enhance nutrients in food crops. The purpose of the present study was to fortify tomato fruits with vitamin B12 (or cyanocobalamin). RESULTS: Tomato plants were grown for 70 days in hydroponic culture pots and treated with 5 µm of cyanocobalamin on days 1-24 after the fruiting, and then harvested for tomato fruits. The ripened tomato fruits contained 4.0 × 10-7  g of cyanocobalamin per 100 g of dry weight and showed a significant increase in glucose and lycopene levels. CONCLUSION: The present study highlights the use of a cyanocobalamin-supplementation system for the production of B12 fortified tomato fruits that can help prevent B12 deficiency in vegetarians. © 2022 Society of Chemical Industry.

Silica adsorption tag derived from the silica polycondensation protein glassin for the immobilization of soluble proteins.

Glassin is a water-soluble protein from the siliceous skeleton of a marine sponge that adsorbs tightly to silica at pH 7.0-9.0 and accelerates silica particle formation from silicic acid. Glassin comprises three distinct domains: a His and Asp-rich (HD) domain, a Pro-rich (P) domain, and a His and Thr-rich (HT) domain. Here, we investigated the roles of these three domains in silica adsorption by using glutathione S-transferase (GST) fused with glassin or with each domain. GST fused with the HD domain exhibited tight adsorption, equivalent to that of GST fused with the full-length glassin sequence at values above 7.0. The apparent Kd values for the binding of full-length glassin and HD to fumed silica at pH 7.0 were 20.8 and 22.7 nM, respectively, indicating that this domain greatly contributes to the silica adsorption ability of glassin. In addition, no internal cleavage was observed in the HD domain, whereas GST fused with the full-length glassin sequence exhibited internal cleavage. The HD domain adsorbed on silica did not dissociate even at pH 6.0. Given these findings, we concluded that the HD domain has potential as a tag for the immobilization of soluble proteins.

Chemicals behind the use of Strombus tricornis opercula in traditional sudanese perfumery and medicine.

Strombidae is one of the major molluscan families in Sudan and due to their opercula, has tremendous economic value. In traditional Sudanese homemade perfumes and body care cosmetics, Strombidae family operculum is one of the main ingredients. Their fumigation generates a charming odor preferred by Sudanese people, used for body smoke baths by married women. Moreover, these fumes are believed to treat several gynecological disorders. In this study, we attempted to confirm the presence of volatiles with pleasant odors and compounds with pharmaceutical importance in the Strombidae opercula. Volatiles from the smoke and soak extracts of the burned opercula were analyzed using gas chromatography-mass spectrometry (GC-MS). Furthermore, polar components from the methanol extract of opercula powder were isolated using high-performance liquid chromatography (HPLC) and identified by nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS), and UV spectra. The elemental and metal contents were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). GC-MS analysis revealed several phenols, aldehydes, ketones, and other functional fragrant and volatile constituents. Further, two compounds were purified from the methanol extract of Strombidae opercula, and named compounds B and D, which were identified as cyclo-(Tyr-Gly) and 4-hydroxybenzaldehyde, respectively. ICP-MS analysis revealed the presence of various elements and metals at different levels. These findings support the historical and traditional practices and usage of the Strombidae opercula in therapeutic and esthetic products. The opercula contains many biologically active compounds and produces smoke containing volatile scent compounds, which might provide alternative pharmaceuticals and cosmetic ingredients that can cooperate to improve the manufacturing of numerous medical products.

Argeloside I inhibits the pathogenicity of Porphyromonas gingivalis TDC60.

Porphyromonas gingivalis, a major pathogen associated with chronic periodontitis, produces several virulence agents in the outer cell membrane, including gingipains and hemagglutinins. These virulence factors enable the bacteria to adhere to periodontal tissue and degrade host proteins to obtain the nutrients needed for dental plaque formation. P. gingivalis TDC60 was recently identified as the most aggressive P. gingivalis strain to dates. In this study, we isolated a known pregnane glycoside, argeloside I, from the aqueous extract of Solenostemma argel leaves. Argeloside I completely hindered the growth of P. gingivalis TDC60 and inhibited the production of hemagglutinins as well as Arg- and Lys-specific gingipains. Our results demonstrate a new function of pregnane glycosides. Argeloside I may be a candidate for reducing the risk associated with P. gingivalis TDC60 and its adhesion factors.

Identification of the Domains Involved in Promotion of Silica Formation in Glassin, a Protein Occluded in Hexactinellid Sponge Biosilica, for Development of a Tag for Purification and Immobilization of Recombinant Proteins.

Glassin, a protein occluded in biosilica of the hexactinellid sponge Euplectela, promotes silica formation from silicic acid at room temperature and neutral pH and is characterized by its primary structure which consists of a tandem repeat carrying three distinct domains, histidine and aspartic acid-rich (HD) domain, proline-rich (P) domain, and histidine and threonine-rich (HT) domain. The present study aims to clarify the domain responsible for the promotion of silica formation and to demonstrate usefulness of glassin and its domain as a tag for purification and immobilization of recombinant proteins. When each domain was mixed with silicic acid at neutral pH, silica was formed with HD domain as well as glassin, or a single repeat, but not with P or HT domain. Neither of amino or carboxy-terminal half of HD domain induced silica formation. The amount of silica formed with HD domain was significantly lower than that of glassin or a single repeat. HD domain fused with HT domain raised the amount of silica formed, while a HD domain fused with P domain, a mixture of HD and P domains, or a mixture of HD and HT domains has little effect on the promotion of silica formation. Collectively, a minimum sequence for promotion of silica formation is HD domain, whose activity can be enhanced by HT domain through a topological effect. In addition, practicality of glassin and HD domain was demonstrated by fusion of these sequences to green fluorescent protein which was successfully purified with Ni affinity chromatography and immobilized on silica.

Chitinolytic proteins secreted by Cellulosimicrobium sp. NTK2.

Cellulosimicrobium sp. NTK2 (NTK2 strain) was isolated as a chitinolytic bacterium from mature compost derived from chitinous waste. The growth of the NTK2 strain was enhanced by supplementation of the culture medium with 2% crystalline chitin. Approximately 70% of the supplemented crystalline chitin was degraded during cultivation. Whole genome analysis of the NTK2 strain identified eight chitinases and two chitin-binding proteins. The NTK2 strain secreted two bacterial extracellular solute-binding proteins, three family 18 glycosyl hydrolases and one lytic polysaccharide monooxygenase specifically in the presence of crystalline chitin. A chitinolytic enzyme with a molecular mass of 29 kDa on SDS-PAGE under native conditions was also secreted. This chitinolytic enzyme exhibited the largest band upon zymography but could not be identified. In an attempt to identify all the chitinases secreted by the NTK2 strain, we expressed recombinant versions of the proteins exhibiting chitinolytic activity in Escherichia coli. Our results suggest that the 29 kDa protein belonging to family 19 glycosyl hydrolase was expressed specifically in the presence of 2% crystalline chitin.

Involvement of Spermidine in the Reduced Lifespan of Caenorhabditis elegans During Vitamin B12 Deficiency.

Vitamin B12 deficiency leads to various symptoms such as neuropathy, growth retardation, and infertility. Vitamin B12 functions as a coenzyme for two enzymes involved in amino acid metabolisms. However, there is limited information available on whether amino acid disorders caused by vitamin B12 deficiency induce such symptoms. First, free amino acid levels were determined in vitamin B12-deficient Caenorhabditis elegans to clarify the mechanisms underlying the symptoms caused by vitamin B12 deficiency. Various amino acids (valine, leucine, isoleucine, methionine, and cystathionine, among others) metabolized by vitamin B12-dependent enzymes were found to be significantly changed during conditions of B12 deficiency, which indirectly affected certain amino acids metabolized by vitamin B12-independent enzymes. For example, ornithine was significantly increased during vitamin B12 deficiency, which also significantly increased arginase activity. The accumulation of ornithine during vitamin B12 deficiency constitutes the first report. In addition, the biosynthesis of spermidine from ornithine was significantly decreased during vitamin B12 deficiency, likely due to the reduction of S-adenosylmethionine as a substrate for S-adenosylmethionine decarboxylase, which catalyzes the formation of spermidine. Moreover, vitamin B12 deficiency also demonstrated a significant reduction in worm lifespan, which was partially recovered by the addition of spermidine. Collectively, our findings suggest that decreased spermidine is one factor responsible for reduced lifespan in vitamin B12-deficient worms.

Isolates from Monechma ciliatum seeds' extract hampered Porphyromonas gingivalis hemagglutinins.

Porphyromonas gingivalis is a major periodontitis pathogen that produces several virulence factors including hemagglutinins. These proteins, which are vital molecules, allow P. gingivalis to uptake iron and heme by attaching, aggregating, and lysing erythrocytes. In this study, we evaluated the inhibitory activity of the aqueous extract of Monechma ciliatum seeds against the hemagglutination activity of P. gingivalis. M. ciliatum is a Sudanese medicinal herb that grows in arid and semi-arid lands of tropical Africa. The water extracted from dry powdered seeds was partitioned using ethyl acetate followed by reversed-phase chromatography, thin-layer chromatography, ESI-MS, and NMR analysis resulting in the isolation of four compounds identified as oleic acid, coumarin, 1,2-dioleoylglycerol, and 1,3-dioleoylglycerol with MICs of 15-100 µg/ml against hemagglutination. We believe that the isolation and purification of these compounds will expand the application of M. ciliatum as a natural therapeutic or preventative agent. PRACTICAL APPLICATIONS: Monechma ciliatum or black mahlab is a famous medicinal plant that grows in some parts of arid and semi-arid areas of tropical Africa including western Sudan. Despite its nutritional and traditional medical applications, no studies have evaluated its anti-hemagglutination activity against periodontal pathogens. In this study, four active compounds (oleic acid, coumarin, 1,2-dioleoylglycerol, and 1,3-dioleoylglycerol) were isolated and identified from an aqueous extract of M. ciliatum seeds. The isolated compounds revealed high levels of inhibitory activity against all hemagglutinin agents secreted by Porphyromonas gingivalis. This evidence of inhibitory activity will encourage the application of M. ciliatum effectively as a functional food or therapeutic agent to prevent periodontal diseases in the early stages.

Effect of Active Site Pocket Structure Modification of D-Stereospecific Amidohydrolase on the Recognition of Stereospecific and Hydrophobic Substrates.

D-Stereospecific amidohydrolase (DAH) from Streptomyces sp. 82F2 has potential utility for the synthesis of D/L configuration dipeptides by an aminolysis reaction. Structural comparison of DAH with substrate-bound D-amino acid amidase revealed that three residues located in the active site pocket of DAH (Thr145, Ala267, and Gly271) might be involved in interactions with D-phenylalanine substrate. We substituted Ala267 and Gly271, which are located at the bottom of the hydrophobic pocket of DAH, with Phe and observed changes in the stereoselectivity and specific activity toward the free and acetylated forms of D/L-Phe-methyl esters. In contrast, the mutation of Thr145, which likely supplies negative charge for recognition of the amino group of the substrate, hardly affected the stereoselectivity of the enzyme. A similar effect was observed in an investigation of hydrolysis and aminolysis reactions using the acetylated forms of D/L-Phe-methyl esters and 1,8-diaminooctane as an acyl-donor and acyl-acceptor, respectively. Substrate binding by DAH was disrupted by the mutation of Ala267 to Val or Trp and kinetic analysis showed that the hydrophobicity of the bottom of the active site pocket (Ala267 and Gly271) is important for both stereoselectivity and recognition of hydrophobic substrates.

Active site pocket of Streptomycesd-stereospecific amidohydrolase has functional roles in aminolysis activity.

d-Stereospecific amidohydrolase from Streptomyces sp. 82F2 (DAH) recognizes d-amino acyl ester derivatives as substrates and catalyzes hydrolysis and aminolysis to yield d-amino acids and d-amino acyl peptides or amide derivatives, respectively. Crystallographic analysis has revealed that DAH possesses a large cavity with a small pocket at the bottom. Because the pocket is close to the catalytic center and is thought to interact with substrates, we examined the function of the eight residues that form the pocket in terms of substrate recognition and aminolysis via mutational analysis. Formation of the acyl-enzyme intermediate and catalysis of aminolysis by DAH were changed by substitutions of selected residues with Ala. In particular, I338A DAH exhibited a significant increase in the condensation product of Ac-d-Phe methyl ester and 1,8-diaminooctane (Ac-d-Phe-1,8-diaminooctane) compared with the wild-type DAH. A similar effect was observed by the mutation of Ile338 to Gly and Ser. The pocket shapes and local flexibility of the mutants I338G, I338A, and I338S are thought to resemble each other. Thus, changes in the shape and local flexibility of the pocket of DAH by mutation presumably alter substrate recognition for aminolysis.

Methionine residues lining the substrate pathway in prolyl oligopeptidase from Pleurotus eryngii play an important role in substrate recognition.

Family S9 prolyl oligopeptidases (POPs) are of interest as pharmacological targets. We recently found that an S9 POP from Pleurotus eryngii showed altered substrate specificity following H2O2 treatment. Oxidation of Met203 on the non-catalytic ß-propeller domain resulted in decreased activity toward non-aromatic aminoacyl-para-nitroanilides (pNAs) while maintaining its activity toward aromatic aminoacyl-pNAs. Given that the other Met residues should also be oxidized by H2O2 treatment, we constructed mutants in which all the Met residues were substituted with other amino acids. Analysis of the mutants showed that Met570 in the catalytic domain is another potent residue for the altered substrate specificity following oxidation. Met203 and Met570 lie on the surfaces of two different domains and form part of a funnel from the surface to the active center. Our findings indicate that the funnel forms the substrate pathway and plays a role in substrate recognition.

Loop of Streptomyces Feruloyl Esterase Plays an Important Role in the Enzyme's Catalyzing the Release of Ferulic Acid from Biomass.

Feruloyl esterases (FAEs) are key enzymes required for the production of ferulic acid from agricultural biomass. Previously, we identified and characterized R18, an FAE from Streptomyces cinnamoneus NBRC 12852, which showed no sequence similarity to the known FAEs. To determine the region involved in its catalytic activity, we constructed chimeric enzymes using R18 and its homolog (TH2-18) from S. cinnamoneus strain TH-2. Although R18 and TH2-18 showed 74% identity in their primary sequences, the recombinant proteins of these two FAEs (recombinant R18 [rR18] and rTH2-18) showed very different specific activities toward ethyl ferulate. By comparing the catalytic activities of the chimeras, a domain comprised of residues 140 to 154 was found to be crucial for the catalytic activity of R18. Furthermore, we analyzed the crystal structure of rR18 at a resolution of 1.5 Å to elucidate the relationship between its activity and its structure. rR18 possessed a typical catalytic triad, consisting of Ser-191, Asp-214, and His-268, which was characteristic of the serine esterase family. By structural analysis, the above-described domain was found to be present in a loop-like structure (the R18 loop), which possessed a disulfide bond conserved in the genus Streptomyces Moreover, compared to rTH2-18 of its parental strain, the TH2-18 mutant, in which Pro and Gly residues were inserted into the domain responsible for forming the R18 loop, showed markedly high kcat values using artificial substrates. We also showed that the FAE activity of TH2-18 toward corn bran, a natural substrate, was improved by the insertion of the Gly and Pro residues.IMPORTANCEStreptomyces species are widely distributed bacteria that are predominantly present in soil and function as decomposers in natural environments. They produce various enzymes, such as carbohydrate hydrolases, esterases, and peptidases, which decompose agricultural biomass. In this study, based on the genetic information on two Streptomyces cinnamoneus strains, we identified novel feruloyl esterases (FAEs) capable of producing ferulic acid from biomass. These two FAEs shared high similarity in their amino acid sequences but did not resemblance any known FAEs. By comparing chimeric proteins and performing crystal structure analysis, we confirmed that a flexible loop was important for the catalytic activity of Streptomyces FAEs. Furthermore, we determined that the catalytic activity of one FAE was improved drastically by inserting only 2 amino acids into its loop-forming domain. Thus, differences in the amino acid sequence of the loop resulted in different catalytic activities. In conclusion, our findings provide a foundation for the development of novel enzymes for industrial use.

Effect of oxidation of the non-catalytic ß-propeller domain on the substrate specificity of prolyl oligopeptidase from Pleurotus eryngii.

Enzymes belonging to the S9 family of prolyl oligopeptidases are of interest because of their pharmacological importance and have a non-catalytic ß-propeller domain. In this study, we found that the oxidation of Met203, which lies on surface of the ß-propeller domain, leads to change in the substrate specificity of eryngase, an enzyme from Pleurotus eryngii and a member of the S9 family of prolyl oligopeptidases. The activity of eryngase for L-Phe-p-nitroanilide was maintained following hydrogen peroxide treatment but was dramatically reduced for other p-nitroanilide substrates. MALDI-TOF MS analysis using tryptic peptides of eryngase indicated that the change in substrate specificity was triggered by oxidizing Met203 to methionine sulfoxide. In addition, mutations of Met203 to smaller residues provided specificities similar to those observed following oxidation of the wild-type enzyme. Substitution of Met203 with Phe significantly decreased activity, indicating that Met203 may be involved in substrate gating.

Purification, gene cloning, and characterization of γ-butyrobetainyl CoA synthetase from Agrobacterium sp. 525a.

The report is the first of purification, overproduction, and characterization of a unique γ-butyrobetainyl CoA synthetase from soil-isolated Agrobacterium sp. 525a. The primary structure of the enzyme shares 70-95% identity with those of ATP-dependent microbial acyl-CoA synthetases of the Rhizobiaceae family. As distinctive characteristics of the enzyme of this study, ADP was released in the catalytic reaction process, whereas many acyl CoA synthetases are annotated as an AMP-forming enzyme. The apparent Km values for γ-butyrobetaine, CoA, and ATP were, respectively, 0.69, 0.02, and 0.24 mM.

Purification and characterization of 4-N-trimethylamino-1-butanol dehydrogenase from Fusarium merismoides var. acetilereum.

From investigation of 60 filamentous fungi, we identified Fusarium merismoides var. acetilereum, which uses 4-N-trimethylamino-1-butanol (TMA-butanol) as the sole source of carbon and nitrogen. The fungus produced NAD(+)-dependent TMA-butanol dehydrogenase (DH) when it was cultivated in medium containing TMA-butanol. The enzyme showed molecular mass of 40 kDa by SDS-PAGE and 160 kDa by gel filtration, suggesting that it is a homotetramer. TMA-butanol DH is stable at pH 7.5-9.0. It exhibits moderate stability with respect to temperature (up to 30 °C). Additionally, it has optimum activity at 45 °C and at pH 9.5. The enzyme has broad specificity to various alkyl alcohols and amino alkyl alcohols, and the carbon chains of which are longer than butanol. Moreover, the activity is strongly inhibited by oxidizing agents, carbonyl and thiol modulators, and chelating agents. This report is the first study examining TMA-butanol DH from eukaryotic microbes.

Crystal structure of D-stereospecific amidohydrolase from Streptomyces sp. 82F2 - insight into the structural factors for substrate specificity.

UNLABELLED: D-Stereospecific amidohydrolase (DAH) from Streptomyces sp. 82F2, which catalyzes amide bond formation from d-aminoacyl esters and l-amino acids (aminolysis), can be used to synthesize short peptides with a dl-configuration. We found that DAH can use 1,8-diaminooctane and other amino compounds as acyl acceptors in the aminolysis reaction. Low concentrations of 1,8-diaminooctane inhibited acyl-DAH intermediate formation. By contrast, excess 1,8-diaminooctane promoted aminolysis by DAH, producing d-Phe-1,8-diaminooctane via nucleophilic attack of the diamine on enzyme-bound d-Phe. To clarify the mechanism of substrate specificity and amide bond formation by DAH, the crystal structure of the enzyme that binds 1,8-diaminooctane was determined at a resolution of 1.49 Å. Comparison of the DAH crystal structure with those of other members of the S12 peptidase family indicated that the substrate specificity of DAH arises from its active site structure. The 1,8-diaminooctane molecule binds at the entrance of the active site pocket. The electrkon density map showed that another potential 1,8-diaminooctane binding site, probably with lower affinity, is present close to the active site. The enzyme kinetics and structural comparisons suggest that the location of enzyme-bound diamine can explain the inhibition of the acyl-enzyme intermediate formation, although the bound diamine is too far from the active site for aminolysis. Despite difficulty in locating the diamine binding site for aminolysis definitively, we propose that the excess diamine also binds at or near the second binding site to attack the acyl-enzyme intermediate during aminolysis. DATABASE: The coordinates and structure factors for d-stereospecific amidohydrolase have been deposited in the Protein Data Bank at the Research Collaboratory for Structural Bioinformatics under code: 3WWX.

Glassin, a histidine-rich protein from the siliceous skeletal system of the marine sponge Euplectella, directs silica polycondensation.

The hexactinellids are a diverse group of predominantly deep sea sponges that synthesize elaborate fibrous skeletal systems of amorphous hydrated silica. As a representative example, members of the genus Euplectella have proved to be useful model systems for investigating structure-function relationships in these hierarchically ordered siliceous network-like composites. Despite recent advances in understanding the mechanistic origins of damage tolerance in these complex skeletal systems, the details of their synthesis have remained largely unexplored. Here, we describe a previously unidentified protein, named "glassin," the main constituent in the water-soluble fraction of the demineralized skeletal elements of Euplectella. When combined with silicic acid solutions, glassin rapidly accelerates silica polycondensation over a pH range of 6-8. Glassin is characterized by high histidine content, and cDNA sequence analysis reveals that glassin shares no significant similarity with any other known proteins. The deduced amino acid sequence reveals that glassin consists of two similar histidine-rich domains and a connecting domain. Each of the histidine-rich domains is composed of three segments: an amino-terminal histidine and aspartic acid-rich sequence, a proline-rich sequence in the middle, and a histidine and threonine-rich sequence at the carboxyl terminus. Histidine always forms HX or HHX repeats, in which most of X positions are occupied by glycine, aspartic acid, or threonine. Recombinant glassin reproduces the silica precipitation activity observed in the native proteins. The highly modular composition of glassin, composed of imidazole, acidic, and hydroxyl residues, favors silica polycondensation and provides insights into the molecular mechanisms of skeletal formation in hexactinellid sponges.