A mutant of Prevotella ruminicola B14 deficient in β-1,4-endoglucanase and mannanase activities

Transcriptional regulation of beta-glucanase activity in the ruminal bacterium, Prevotella bryantii B14

The ruminal bacterium, Prevotella bryantii B(1)4, grew more rapidly with glucose as an energy source than mannose (0.73 versus 0.47 h(-1)) and had 8-fold less beta-glucanase activity (50 versus 400 nmol reducing sugar mg protein(-1) min(-1)). Cultures that were provided with glucose and mannose had little beta-glucanase activity even though both sugars were utilized simultaneously. The observation that glucose and mannose were utilized simultaneously indicated that beta-glucanase expression was not merely a simple induction or inducer exclusion. When glucose was added to cultures growing on mannose, hexose flux through the glucomannokinase increased 1.5-fold, and this increase was associated with an almost immediate decrease in beta-glucanase mRNA. After only three generation (doubling) times, the amount of beta-glucanase mRNA was comparable to that observed in cells growing only with glucose. These results indicate that beta-glucanase activity is transcriptionally regulated. However, further work will be needed to define more precisely the nature of this regulation and to identify the intermediate in this response.

The glucomannokinase of Prevotella bryantii B(1)4 and its potential role in regulating beta-glucanase expression

Prevotella bryantii B(1)4 has a transport system for glucose and mannose, but beta-glucanase expression is only catabolite-repressed by glucose. P bryantii B(1)4 cell extracts had ATP-dependent gluco- and mannokinase activities, and significant phosphoenolpyruvate- or GTP-dependent hexose phosphorylation was not observed. Mannose inhibited glucose phosphorylation (and vice versa), and activity gels indicated that a single protein was responsible for both activities. Glucose was phosphorylated at a faster rate than was mannose [V(max) 280 nmol hexose (mg protein)(-1) min(-1) versus 60 nmol hexose (mg protein)(-1) min(-1), respectively] and glucose was a better substrate for the kinase (K(m) 0.12 mM versus 1.2 mM, respectively). The purified glucomannokinase (1250-fold) had a molecular mass of 68 kDa, but SDS-PAGE gels indicated that it was a dimer (monomer 34.5 kDa). The N-terminus (25 residues) had an 8 amino acid segment that was homologous to other bacterial glucokinases. The glucomannokinase was competitively inhibited by the nonmetabolizable glucose analogue 2-deoxyglucose (2DG), and cells grown with glucose and 2DG had lower rates of glucose consumption than did cells given only glucose. When the ratio of 2DG to glucose was increased, the glucose consumption rate decreased and the beta-glucanase activity increased. The glucose consumption rate and the glucomannokinase activity of cells treated with 2DG were highly correlated (r(2)=0.98). This result suggested that glucomannokinase activity was either directly or indirectly regulating beta-glucanase expression.

Alternative pathways of glucose transport in Prevotella bryantii B(1)4(1)

Prevotella bryantii B(1)4 grew faster on glucose than mannose (0.70 versus 0.45 h(-1)), but these sugars were used simultaneously rather than diauxically. 2-deoxy-glucose (2DG) decreased the growth rate of cells that were provided with either glucose or mannose, but 2DG did not completely prevent growth. Cells grown on glucose or mannose transported both (14)C-glucose and (14)C-mannose, but cells grown on glucose had over three-fold higher rates of (14)C-glucose transport than cells grown on mannose. The (14)C-mannose transport rates of glucose- and mannose-grown cells were similar. Woolf-Augustinsson-Hofstee plots were not linear, and it appeared that the glucose/mannose/2DG carrier acted as a facilitated diffusion system at high substrate concentrations. When cultures were grown on nitrogen-deficient (excess sugar) medium, isolates had three-fold lower (14)C-glucose transport, but the (14)C-mannose transport did not change significantly. (14)C-glucose and (14)C-mannose transport rates could be inhibited by 2DG and either mannose or glucose, respectively. The (14)C-glucose transport of mannose-grown cells was inhibited more strongly by mannose and 2DG than those grown on glucose. Cells grown on glucose or mannose had similar ATP-dependent glucokinase activity, and 2DG was a competitive inhibitor (K(i)=0.75 mM). Thin layer chromatography indicated that cell extracts also had ATP-dependent mannose phosphorylation, but only a small amount of phosphorylated 2DG was detected. Glucose, mannose or 2DG were not phosphorylated in the presence of PEP. Based on these results, it appeared that P. bryantii B(1)4 had: (1) two mechanisms of glucose transport, a constitutive glucose/mannose/2DG carrier and an alternative glucose carrier that was regulated by glucose availability, (2) an ATP-dependent glucokinase that was competitively inhibited by 2DG but was unable to phosphorylate 2DG at a rapid rate, and (3) virtually no PEP-dependent glucose, mannose or 2DG phosphorylation activities.