Lactococcus lactis as a cell factory: a twofold increase in phosphofructokinase activity results in a proportional increase in specific rates of glucose uptake and lactate formation.

Despite the fact that the area of glycolysis in Lactococcus lactis has been intensively studied, only a limited number of studies have been focused on the regulation of uptake of glucose itself. Using the tool of the glucostat fed-batch mode of culture, it was demonstrated in our earlier work that the concentration of glucose regulates its uptake rate and that the control of the glycolytic flux resides to a large extent in processes outside the pathway itself, like glucose transport and the ATP consuming reactions, while allosteric properties of key enzymes like phosphofructokinase (PFK) have a significant influence on the control. Extending our work, we report here the results of fermentations with engineered L. lactis strains with altered PFK activity in which the pfkA gene from Aspergillus niger, and its truncated version pfk13 that encodes a shorter PFK1 fragment were cloned. The results in this study suggest that, under the optimum for the microorganism applied microaerobic conditions, the glycolytic capacity of L. lactis was significantly increased in engineered strains with increased PFK activity. The transformant strain in which the truncated pfk13 gene of A. niger was expressed performed more efficiently as it was able to grow successfully in glucostat cultures with 277 mM glucose - while the optimum glucose concentration for the parental strain was 55 mM. The present work demonstrates the direct effect of PFK activity on the glycolytic flux in L. lactis since a twofold increase in specific PFK activity (from 7.1 to 14.5 U/OD(600)) resulted in a proportional increase of the maximum specific rates of glucose uptake (from 0.8 to 1.7 μMs(-1) g CDW(-1)) and lactate formation (from 15 to 22.8 g lactate (g CDW)(-1) h(-1)).