Molecular characterization and secreted production of basidiomycetous cell-bound ß-glycosidases applicable to production of galactooligosaccharides.

Cell-bound ß-glycosidases of basidiomycetous yeasts show promise as biocatalysts in galactooligosaccharide (GOS) production. Using degenerated primers designed from Hamamotoa singularis (Hs) bglA gene, we newly identified three genes that encode cell-bound ß-glycosidase from Sirobasidium magnum (Sm), Rhodotorula minuta (Rm), and Sterigmatomyces elviae (Se). These three genes, also named bglA, encoded family 1 glycosyl hydrolases with molecular masses of 67‒77 kDa. The BglA enzymes were approximately 44% identical to the Hs-BglA enzyme and possessed a unique domain at the N-terminus comprising 110 or 210 amino acids. The Sm-, Rm-, and Se-BglA enzymes as well as the Hs-BglA enzyme were successfully produced by recombinant Aspergillus oryzae, and all enzymes were entirely secreted to the supernatants. Furthermore, addition of some nonionic detergents (e.g. 0.4% [v/v] Triton-X) increased the production, especially of the Hs- or Se-BglA enzyme. Out of the BglA enzymes, the Se-BglA enzyme showed remarkable thermostability (∼70°C). Additionally, the Sm- and Se-BglA enzymes had better GOS yields, so there was less residual lactose than in others. Accordingly, the basidiomycetous BglA enzymes produced by recombinant A. oryzae would be applicable to GOS production, and the Se-BglA enzyme appeared to be the most promising enzyme for industrial uses.

Identification of genes involved in galactooligosaccharide utilization in Bifidobacterium breve strain YIT 4014T.

Galactooligosaccharides (GOS) are mixed oligosaccharides that are mainly composed of galactosyllactoses (GLs), which include 3'-GL, 4'-GL, and 6'-GL. Data from numerous in vitro and in vivo studies have shown that GOS selectively stimulate the growth of bifidobacteria. Previously, we identified the gene locus responsible for 4'-GL utilization, but the selective routes of uptake and catabolism of 3'- and 6'-GL remain to be elucidated. In this study, we used differential transcriptomics to identify the utilization pathways of these GLs within the Bifidobacterium breve YIT 4014T strain. We found that the BBBR_RS 2305-2320 gene locus, which includes a solute-binding protein (SBP) of an ATP-binding cassette (ABC) transporter and ß-galactosidase, were up-regulated during 3'- and 6'-GL utilization. The substrate specificities of these proteins were further investigated, revealing that ß-galactosidase hydrolyzed both 3'-GL and 6'-GL efficiently. Our surface plasmon resonance results indicated that the SBP bound strongly to 6'-GL, but bound less tightly to 3'-GL. Therefore, we looked for the other SBPs for 3'-GL and found that the BBBR_RS08090 SBP may participate in 3'-GL transportation. We also investigated the distribution of these genes in 17 bifidobacterial strains, including 9 B. breve strains, and found that the ß-galactosidase genes were present in most bifidobacteria. Homologues of two ABC transporter SBP genes were found in all B. breve strains and in some bifidobacteria that are commonly present in the human gut microbiota. These results provide insights into the ability of human-resident bifidobacteria to utilize the main component of GOS in the gastrointestinal tract.

Characterization of a bifidobacterial system that utilizes galacto-oligosaccharides.

The galacto-oligosaccharide (GOS) OLIGOMATE 55N (Yakult) is a mixture of oligosaccharides, the main component of which is 4'-galactosyllactose (4'-GL). Numerous reports have shown that GOSs are non-digestible, reach the colon and selectively stimulate the growth of bifidobacteria. The product has been used as a food ingredient and its applications have expanded rapidly. However, the bifidobacterial glycoside hydrolases and transporters responsible for utilizing GOSs have not been characterized sufficiently. In this study, we aimed to identify and characterize genes responsible for metabolizing 4'-GL in Bifidobacterium breve strain Yakult. We attempted to identify B. breve Yakult genes induced by 4'-GL using transcriptional profiling during growth in basal medium containing 4'-GL with a custom microarray. We found that BbrY_0420, which encodes solute-binding protein (SBP), and BbrY_0422, which encodes ß-galactosidase, were markedly upregulated relative to that during growth in basal medium containing lactose. Investigation of the substrate specificity of recombinant BbrY_0420 protein using surface plasmon resonance showed that BbrY_0420 protein bound to 4'-GL, but not to 3'-GL and 6'-GL, structural isomers of 4'-GL. Additionally, BbrY_0420 had a strong affinity for 4-galactobiose (4-GB), suggesting that this SBP recognized the non-reducing terminal structure of 4'-GL. Incubation of purified recombinant BbrY_0422 protein with 4'-GL, 3'-GL, 6'-GL and 4-GB revealed that the protein efficiently hydrolysed 4'-GL and 4-GB, but did not digest 3'-GL, 6'-GL or lactose, suggesting that BbrY_0422 digested the bond within Gal1,4-ß-Gal. Thus, BbrY_0420 (SBP) and BbrY_0422 (ß-galactosidase) had identical, strict substrate specificity, suggesting that they were coupled by co-induction to facilitate the transportation and hydrolysis of 4'-GL.

Method for direct discrimination of intra- and intermolecular hydrogen bonds, and characterization of the G(:A):G(:A):G(:A):G heptad, with scalar couplings across hydrogen bonds.

Discrimination of intra- and intermolecular hydrogen bonds in a symmetric multimer has not been accomplished yet, although such discrimination would provide a crucial basis for construction of the multimeric architecture of nucleic acids by NMR. We have developed a direct and unambiguous method for such discrimination involving the use of scalar couplings across hydrogen bonds. The method has been validated with a symmetric dimer of d(GGGCTTTTGGGC), for which the structure including both intra- and intermolecular hydrogen bonds was already reported. This has demonstrated that our method can clearly discriminate these two kinds of hydrogen bonds. Then, the method was applied to a symmetric dimer of d(GGAGGAGGAGGA) and has provided decisive information on its multimeric architecture. Additionally, the values for scalar couplings across hydrogen bonds for G:G and G:A base pairs in the G(:A):G(:A):G(:A):G heptad formed by d(GGAGGAGGAGGA) were determined for the first time. This determination has provided an insight into the nature of the heptad.

Unique quadruplex structures of d(GGA)4 (12-mer) and d(GGA)8 (24-mer)--direct evidence of the formation of non-canonical base pairs and structural comparison.

We have reported that d(GGA)4 (12-mer) folds into an intramolecular parallel quadruplex with a G:G:G:G tetrad and a G(:A):G(:A):G(:A):G heptad and that two quadruplexes form a dimer. Here we present the unique structure of d(GGA)8 (24-mer) under physiological conditions. G1-A12 and G13-A24 segments of d(GGA)8 fold into an intramolecular parallel quadruplex with the tetrad and heptad, respectively, and two quadruplexes stack each other in a head-to-head manner. The formation of non-canonical G:G and G:A base pairs was directly certified by observing spin-spin couplings across hydrogen bonds with the aid of 13C-, 15N-labelling of DNA. It was noted that the monomeric architecture of d(GGA)8 resembles the dimeric architecture of d(GGA)4. Interestingly, the recent X-ray study shows that the telomere DNA also folds into the similar intramolecular parallel quadruplex as we have reported for GGA-repeat DNAs.

Structure, stability and application to functional DNA/RNA of unique quadruplex.

We have developed a direct and unambiguous method for discrimination of intra- and intermolecular hydrogen bonds in a symmetric multimer. The method was applied to a symmetric dimer of d(GGAGGAGGAGGA) (GGA 12-mer) and has provided decisive information on its multimeric architecture. Then, the values for scalar couplings across hydrogen bonds for G:G and G:A base pairs in the G(:A):G(:A):G(:A):G heptad formed by GGA 12-mer were determined. This determination has provided an insight into the stability of the heptad. Moreover, GGA 12-mer has been incorporated into functional DNA, and unique structural features of GGA 12-mer have been successfully utilized to regulate the activity of functional DNA.