Degradation of rice bran hemicellulose by Paenibacillus sp. strain HC1: gene cloning, characterization and function of β-D-glucosidase as an enzyme involved in degradation

A bacterium (strain HC1) capable of assimilating rice bran hemicellulose was isolated from a soil and identified as belonging to the genus Paenibacillus through taxonomical and 16S rDNA sequence analysis. Strain HC1 cells grown on rice bran hemicellulose as a sole carbon source inducibly produced extracellular xylanase and intracellular glycosidases such as β-d-glucosidase and β-d-arabinosidase. One of them, β-d-glucosidase was further analyzed. A genomic DNA library of the bacterium was constructed in Escherichia coli and gene coding for β-d-glucosidase was cloned by screening for β-d-glucoside-degrading phenotype in E. coli cells. Nucleotide sequence determination indicated that the gene for the enzyme contained an open reading frame consisting of 1,347 bp coding for a polypeptide with a molecular mass of 51.4 kDa. The polypeptide exhibits significant homology with other bacterial β-d-glucosidases and belongs to glycoside hydrolase family 1. β-d-Glucosidase purified from E. coli cells was a monomeric enzyme with a molecular mass of 50 kDa most active at around pH 7.0 and 37°C. Strain HC1 glycosidases responsible for degradation of rice bran hemicellulose are expected to be useful for structurally determining and molecularly modifying rice bran hemicellulose and its derivatives.