Digestive β-glucosidases from the wood-feeding higher termite, Nasutitermes takasagoensis: Intestinal distribution, molecular characterization, and alteration in sites of expression

β-Glucosidase [EC 3.2.1.21] hydrolyzes cellobiose or cello-oligosaccharides into glucose during cellulose digestion in termites. SDS-PAGE and zymogram analyses of the digestive system in the higher termite Nasutitermes takasagoensis revealed that β-glucosidase activity is localized in the salivary glands and midgut as dimeric glycoproteins. Degenerate PCR using primers based on the N-terminal amino acid sequences of the salivary β-glucosidase resulted in cDNA fragments of 1.7 kb, encoding 489 amino acids with a sequence similar to glycosyl hydrolase family 1. Moreover, these primers amplified cDNA fragments from the midgut, and the deduced amino acid sequences are 87-91% identical to those of the salivary β-glucosidases. Successful expression of the cDNAs in Escherichia coli implies that these sequences also encode functional β-glucosidases. These results indicate that β-glucosidases that primarily contribute to the digestive process of N. takasagoensis are produced in the midgut. Reverse transcription-PCR analysis indicated the site-specific expression of β-glucosidase mRNAs in the salivary glands and midgut. These results suggest that termites have developed the ability to produce β-glucosidases in the midgut, as is the case for endo-β-1,4-glucanase, in which the site of expression has shifted from the salivary glands of lower termites to the midgut of higher termites.