CLONING AND SEQUENCING OF A GENE CODING FOR AN EXTRACELLULAR DEXTRANSUCRASE (DSRB) FROM LEUCONOSTOC-MESENTEROIDES NRRL B-1299 SYNTHESIZING ONLY A ALPHA(1-6) GLUCAN

The coding region for a novel Leuconostoc mesenteroides NRRL B-1299 de xtransucrase gene (dsrB) was isolated and sequenced. Using degenerate primers homologous to a conserved region present in dextransucrases fr om Streptococcus (GTFs) and L. mesenteroides NRRL B-512F (DSRS) and co nserved amino acid sequences located in the N-terminal catalytic regio n of these enzymes, about 60% of the DSRB encoding gene was isolated. Two sites, BamHI and HindIII, were identified which allowed one 0.5-kb p probe to be obtained to isolate the 5' and the 3' ends of dsrB. The nucleotide sequence of the dsrB gene was determined and found to consi st of an open reading frame (ORF) of 4521 base pairs (bp) coding for a 1507-amino acid protein with an M-r, of 168 511. The amino acid seque nce is very close to that of DSRS. Like DSRS, the dextran produced app eared to be composed of only alpha(1-6) glucosidic bonds, and the olig osaccharides synthesized in the presence of acceptor maltose were also composed of alpha(1-6) linked glucosyl residues in addition to the ma ltosyl residue. DSRB thus appears to be a novel dextransucrase from L. mesenteroides NRRL B-1299. DSRB produces a dextran different from the typical dextran containing alpha(1-6) and alpha(1-2) linkages produce d when this strain is grown in the presence of sucrose. (C) 1998 Feder ation of European Microbiological Societies. Published by Elsevier Sci ence B.V.