Isolation of a Gene from Leuconostoc citreum B/110-1-2 Encoding a Novel Dextransucrase Enzyme

The amplicon encoding dextransucrase DSR-F from Leuconostoc citreum B/110-1-2, a novel sucrose glucosyltransferase (GTF)-specific for alpha-1,6 and alpha-1,3 glucosidic bond synthesis, with alpha-1,4 branching was cloned, sequenced, and expressed into Escherichia coli JM109. Recombinant enzyme catalyzed oligosaccharides synthesis from sucrose as donor and maltose acceptor. The dsrF gene encodes for a protein (DSR-F) of 1,528 amino acids, with a theoretical molecular mass of 170447.72 Da (similar to 170 kDa). From amino acid sequence comparison, it appears that DSR-F possesses the same domains as those described for GTFs. However, the variable region is longer than in other GTFs (by 100 amino acids) and two APY repeats (a 79 residue long motif with a high number of conserved glycine and aromatic residues, characterized by the presence of the three consecutive residues Ala, Pro, and Tyr) were identified in the glucan binding domain. The DSR-F catalytic domain possesses the catalytic triad involved in the glucosyl enzyme formation. The amino acid sequence of this domain shares a 56% identity with catalytic domain of the alternansucrase ASR from L. citreum NRRL B-1355 and with the catalytic domain of a putative alternansucrase sequence found in the genome of L. citreum KM20. A truncated active variant DSR-F-Delta SP-Delta GBD of 1,251 amino acids, with a molecular mass of 145 544 Da (similar to 145 kDa), was obtained.