A digestive beta-glucosidase from the salivary glands of the termite, Neotermes koshunensis (Shiraki): distribution, characterization and isolation of its precursor cDNA by 5'- and 3'-RACE amplifications with degenerate primers

Parallel metatranscriptome analyses of host and symbiont gene expression in the gut of the termite Reticulitermes flavipes

BACKGROUND

Termite lignocellulose digestion is achieved through a collaboration of host plus prokaryotic and eukaryotic symbionts. In the present work, we took a combined host and symbiont metatranscriptomic approach for investigating the digestive contributions of host and symbiont in the lower termite Reticulitermes flavipes. Our approach consisted of parallel high-throughput sequencing from (i) a host gut cDNA library and (ii) a hindgut symbiont cDNA library. Subsequently, we undertook functional analyses of newly identified phenoloxidases with potential importance as pretreatment enzymes in industrial lignocellulose processing.

RESULTS

Over 10,000 expressed sequence tags (ESTs) were sequenced from the 2 libraries that aligned into 6,555 putative transcripts, including 171 putative lignocellulase genes. Sequence analyses provided insights in two areas. First, a non-overlapping complement of host and symbiont (prokaryotic plus protist) glycohydrolase gene families known to participate in cellulose, hemicellulose, alpha carbohydrate, and chitin degradation were identified. Of these, cellulases are contributed by host plus symbiont genomes, whereas hemicellulases are contributed exclusively by symbiont genomes. Second, a diverse complement of previously unknown genes that encode proteins with homology to lignase, antioxidant, and detoxification enzymes were identified exclusively from the host library (laccase, catalase, peroxidase, superoxide dismutase, carboxylesterase, cytochrome P450). Subsequently, functional analyses of phenoloxidase activity provided results that were strongly consistent with patterns of laccase gene expression. In particular, phenoloxidase activity and laccase gene expression are mostly restricted to symbiont-free foregut plus salivary gland tissues, and phenoloxidase activity is inducible by lignin feeding.

CONCLUSION

To our knowledge, this is the first time that a dual host-symbiont transcriptome sequencing effort has been conducted in a single termite species. This sequence database represents an important new genomic resource for use in further studies of collaborative host-symbiont termite digestion, as well as development of coevolved host and symbiont-derived biocatalysts for use in industrial biomass-to-bioethanol applications. Additionally, this study demonstrates that: (i) phenoloxidase activities are prominent in the R. flavipes gut and are not symbiont derived, (ii) expands the known number of host and symbiont glycosyl hydrolase families in Reticulitermes, and (iii) supports previous models of lignin degradation and host-symbiont collaboration in cellulose/hemicellulose digestion in the termite gut. All sequences in this paper are available publicly with the accession numbers FL634956-FL640828 (Termite Gut library) and FL641015-FL645753 (Symbiont library).

Molecular basis for the reproductive division of labour in a lower termite

Background

Polyphenism, the expression of different phenotypes with the same genetic background, is well known for social insects. The substantial physiological and morphological differences among the castes generally are the result of differential gene expression. In lower termites, workers are developmentally flexible to become neotenic replacement reproductives via a single moult after the death of the founding reproductives. Thus, both castes (neotenics and workers) are expected to differ mainly in the expression of genes linked to reproductive division of labour, which constitutes the fundamental basis of insect societies.

Results

Representational difference analysis of cDNAs was used to study differential gene expression between neotenics and workers in the drywood termite Cryptotermes secundus (Kalotermitidae). We identified and, at least partially cloned five novel genes that were highly expressed in female neotenics. Quantitative real-time PCR analysis of all five genes in different castes (neotenics, founding reproductives, winged sexuals and workers of both sexes) confirmed the differential expression patterns. In addition, the relative expression of these genes was determined in three body parts of female neotenics (head, thorax, and abdomen) using quantitative real-time PCR.

Conclusion

The identified genes could be involved in the control and regulation of reproductive division of labour. Interestingly, this study revealed an expression pattern partly similar to social Hymenoptera indicating both common and species-specific regulatory mechanisms in hemimetabolous and holometabolous social insects.

Correlation of cellulase gene expression and cellulolytic activity throughout the gut of the termite Reticulitermes flavipes

Termites have developed cellulose digestion capabilities that allow them to obtain energy and nutrition from nutritionally poor food sources, such as lignocellulosic plant material and residues derived from it (e.g., wood and humus). Lower termites, which are equipped with both endogenous (i.e., of termite origin) and symbiotic cellulases, feed primarily on wood and wood-related materials. This study investigated cellulase gene diversity, structure, and activity in the lower termite, Reticulitermes flavipes (Kollar). We initially used a metagenomics approach to identify four genes encoding one endogenous and three symbiotic cellulases, which we refer to as Cell-1, -2, -3 and -4. These four genes encode proteins that share significant sequence similarity with known endoglucanases, exoglucanases and xylanases. Phylogenetic analyses further supported these inferred relationships by showing that each of the four cellulase proteins clusters tightly with respective termite, protozoan or fungal cellulases. Gene structure studies revealed that Cell-1, -3 and -4 are intron-free, while Cell-2 contains the first intron sequence to be identified from a termite symbiont cellulase. Quantitative real-time PCR (qRT-PCR) revealed that the endogenous Cell-1 gene is expressed exclusively in the salivary gland/foregut, whereas symbiotic Cell-2, -3, and -4 are highly expressed in the hindgut (where cellulolytic protists are harbored). Cellulase activity assays mapped the distribution pattern of endoglucanase, exoglucanase and xylanase activity throughout the R. flavipes digestive tract. Cellulase gene expression correlated well with the specific types of cellulolytic activities observed in each gut region (foregut+salivary gland, midgut and hindgut). These results suggest the presence of a single unified cellulose digestion system, whereby endogenous and symbiotic cellulases work sequentially and collaboratively across the entire digestive tract of R. flavipes.

Endogenous production of endo-β-1,4-glucanase by decapod crustaceans

The potential ability to produce cellulase enzymes endogenously was examined in decapods crustaceans including the herbivorous gecarcinid land crabs Gecarcoidea natalis and Discoplax hirtipes, the amphibious freshwater crab Austrothelphusa transversa, the terrestrial hermit crab, Coenobita variabilis the parastacid crayfish Euastacus, and the crayfish Cherax destructor. The midgut gland of both G. natalis and D. hirtipes contained substantial total cellulase activities and activities of the cellulase enzymes endo-beta-1,4-glucanase and beta-glucosidase. With the exception of total cellulase and beta-glucosidase from D. hirtipes, the enzyme activities within the midgut gland were higher than those within the digestive juice. Hence, the enzyme activities appear to reside predominantly within midgut gland, providing indirect evidence for endogenous synthesis of cellulase enzymes by this tissue. A 900 bp cDNA fragment encoding a portion of the endo-beta-1,4-glucanase amino acid sequence was amplified by RT-PCR using RNA isolated from the midgut gland of C. destructor, Euastacus, A. transversa and C. variabilis. This provided direct evidence for the endogenous production of endo-beta-1,4-glucanase. The 900 bp fragment was also amplified from genomic DNA isolated from the skeletal muscle of G. natalis and D. hirtipes, clearly indicating that the gene encoding endo-beta-1,4-glucanase is also present in these two species. As this group of evolutionary diverse crustacean species possesses and expresses the endo-beta-1,4-glucanase gene it is likely that decapod crustaceans generally produce cellulases endogenously and are able to digest cellulose.

A digestive β-glucosidase from the silkworm, Bombyx mori: cDNA cloning, expression and enzymatic characterization

A digestive beta-glucosidase cDNA was cloned from the silkworm, Bombyx mori. The B. mori beta-glucosidase cDNA contains an open reading frame of 1473 bp encoding 491 amino acid residues. The B. mori beta-glucosidase possesses the amino acid residues involved in catalysis and substrate binding conserved in glycosyl hydrolase family 1. Southern blot analysis of genomic DNA suggested the B. mori beta-glucosidase to be a single gene. Northern blot analysis of B. mori beta-glucosidase gene confirmed larval midgut-specific expression. The B. mori beta-glucosidase mRNA expression in larval midgut was detectable only during feeding period, whereas its expression was downregulated during starvation. The B. mori beta-glucosidase cDNA was expressed as a 57-kDa polypeptide in baculovirus-infected insect Sf9 cells, and the recombinant beta-glucosidase was active on cellobiose and lactose, but not active on salicin, indicating that the B. mori beta-glucosidase possesses the characteristics of the Class 2 enzyme. The enzyme activity of the purified recombinant beta-glucosidase expressed in baculovirus-infected insect cells was approximately 665 U per microg of recombinant B. mori beta-glucosidase. The purified recombinant B. mori beta-glucosidase showed the highest activity at 35 degrees C and pH 6.0, and were stable at 50 degrees C at least for 10 min. Treatment of recombinant virus-infected Sf9 cells with tunicamycin, a specific inhibitor of N-glycosylation, revealed that the recombinant B. mori beta-glucosidase is N-glycosylated, but the carbohydrate moieties are not essential for enzyme activity.

Major alteration of the expression site of endogenous cellulases in members of an apical termite lineage

Termites are among the most important cellulose-digesting animals on earth, and are well-known for the symbiotic relationship they have with cellulolytic trichomonad and oxymonad flagellates (unicellular eukaryotes). Perhaps less well-known is the fact that approximately 75% of the approximately 2600 described termite species -- those belonging to the family Termitidae -- do not harbour such flagellates. Unlike most termites from other families, the majority of termitids do not consume wood, feeding instead on soil, leaf litter, fungi, grass, or lichen. Recent years have seen the characterization of the endogenous cellulase enzymes that help termites digest cellulose, from one flagellate-harbouring species (Reticulitermes speratus), as well as one termitid (Nasutitermes takasagoensis). The genes encoding the enzymes in these two termites are similar. However, their site of expression differs markedly -- the salivary glands in R. speratus and the midgut in N. takasagoensis. To investigate this difference further, we performed a comparative study of cellulase expression in various termitid and flagellate-harbouring species, using enzyme assays and reverse transcription polymerase chain reactions. Taxa from phylogenetically basal lineages were consistently found to express endogenous genes specifically in the salivary glands, whilst those from a relatively apical lineage containing termitids expressed cellulases solely in the midgut. Relatively low levels of cellulase activity were found in nonwood-feeding species, while the wood-feeding Coptotermes formosanus -- arguably the most destructive pest species world-wide -- was found to have high levels of activity in all parts of the gut when compared to all other termites. In the light of these results, as well as recently accumulated phylogenetic data, we discuss scenarios for the evolution of cellulose digestion in termites.

Purification, characterization, cDNA cloning and nucleotide sequencing of a cellulase from the yellow-spotted longicorn beetle, Psacothea hilaris

A cellulase (endo-beta-1,4-glucanase, EC 3.2.1.4) was purified from the gut of larvae of the yellow-spotted longicorn beetle Psacothea hilaris by acetone precipitation and elution from gels after native PAGE and SDS/PAGE with activity staining. The purified protein formed a single band, and the molecular mass was estimated to be 47 kDa. The purified cellulase degraded carboxymethylcellulose (CMC), insoluble cello-oligosaccharide (average degree of polymerization 34) and soluble cello-oligosaccharides longer than cellotriose, but not crystalline cellulose or cellobiose. The specific activity of the cellulase against CMC was 150 micro mol.min-1.(mg protein)-1. TLC analysis showed that the cellulase produces cellotriose and cellobiose from insoluble cello-oligosaccharides. However, a glucose assay linked with glucose oxidase detected a small amount of glucose, with a productivity of 0.072 micro mol.min-1.(mg protein)-1. The optimal pH of P. hilaris cellulase was 5.5, close to the pH in the midgut of P. hilaris larvae. The N-terminal amino-acid sequence of the purified P. hilaris cellulase was determined and a degenerate primer designed, which enabled a 975-bp cDNA clone containing a typical polyadenylation signal to be obtained by PCR and sequencing. The deduced amino-acid sequence of P. hilaris cellulase showed high homology to members of glycosyl hydrolase family 5 subfamily 2, and, in addition, a signature sequence for family 5 was found. Thus, this is the first report of a family 5 cellulase from arthropods.