Isolation and overexpression of a gene encoding an extracellular beta-(1,3-1,4)-glucanase from Streptococcus bovis JB1.

Streptococcus bovis JB1 was found to produce a 25-kDa extracellular enzyme active against beta-(1,3-1,4)-glucans. A gene was isolated encoding a specific beta-(1,3-1,4)-glucanase that corresponds to this size and belongs to glycoside hydrolase family 16. A 4- to 10-fold increase in supernatant beta-glucanase activity was obtained when the cloned beta-glucanase gene was reintroduced into S. bovis JB1 by use of constructs based on the plasmid vector pTRW10 or pIL253. The beta-(1,3-1,4)-glucanase gene was also expressed upon introduction of the pTRW10 construct pTRWL1R into Lactococcus lactis IL2661 and Enterococcus faecalis JH2-SS, although extracellular activity was 8- to 50-fold lower than that in S. bovis JB1. The beta-(1,3-1,4)-glucanase purified from the culture supernatant of S. bovis JB1 carrying pTRWL1R showed a K(m) of 2.8 mg per ml and a Vmax of 338 mumol of glucose equivalents per min per mg of protein with barley beta-glucan as the substrate. The S. bovis beta-(1,3-1,4)-glucanase may contribute to the ability of this bacterium to utilize starch by degrading structural polysaccharides present in endosperm cell walls.