Identification and Characterization of a Novel Galactofuranose-Specific β-D-Galactofuranosidase from <i>Streptomyces </i>Species

<div><p>β-D-galactofuranose (Gal<i>f</i>) is a component of polysaccharides and glycoconjugates and its transferase has been well analyzed. However, no β-D-galactofuranosidase (Gal<i>f</i>-ase) gene has been identified in any organism. To search for a Gal<i>f</i>-ase gene we screened soil samples and discovered a strain, identified as a <i>Streptomyces</i> species by the 16S ribosomal RNA gene analysis, that exhibits Gal<i>f</i>-ase activity for <i>4</i>-nitrophenyl β-D-galactofuranoside (<i>p</i>NP-β-D-Gal<i>f</i>) in culture supernatants. By draft genome sequencing of the strain, named JHA19, we found four candidate genes encoding Gal<i>f</i>-ases. Using recombinant proteins expressed in <i>Escherichia coli</i>, we found that three out of four candidates displayed the activity of not only Gal<i>f</i>-ase but also α-L-arabinofuranosidase (Ara<i>f</i>-ase), whereas the other one showed only the Gal<i>f</i>-ase activity. This novel Gal<i>f</i>-specific hydrolase is encoded by ORF1110 and has an optimum pH of 5.5 and a Km of 4.4 mM for the substrate <i>p</i>NP-β-D-Gal<i>f</i>. In addition, this enzyme was able to release galactose residue from galactomannan prepared from the filamentous fungus <i>Aspergillus fumigatus</i>, suggesting that natural polysaccharides could be also substrates. By the BLAST search using the amino acid sequence of ORF1110 Gal<i>f</i>-ase, we found that there are homolog genes in both prokaryotes and eukaryotes, indicating that Gal<i>f</i>-specific Gal<i>f</i>-ases widely exist in microorganisms.</p>