Beta-glucose 1-phosphate interconverting enzymes in Lactococcus lactis: physiological role and regulation in maltose, trehalose and glucose metabolism
Sammanfattning: Lactic acid bacteria (LAB), including Lactococcus lactis, are abundant in nature. These bacteria convert carbohydrates into mainly lactate by a rather uncomplicated metabolism. An improved understanding of the physiological and genetical mechanisms involved in carbohydrate metabolism and its regulation will improve the industrial use of LAB. The aim of this study was to investigate the maltose and trehalose assimilating pathways in L. lactis. The maltose phosphorylase (MP) and beta-phosphoglucomutase (b-PGM) of L. lactis were characterised and shown to constitute the major maltose-degrading pathway in this bacterium. Furthermore, MP and b-PGM were shown to be present in many other strains of LAB belonging to the low G+C content LAB of the clostridial sub-branch of gram-positive bacteria. The MP-encoding gene, malP, was localised in an operon distinctive from that of the gene encoding b-PGM, pgmB. In addition, malR, encoding the maltose operon regulator (MalR), was localised downstream of malP. The presence of MalR was shown to be crucial for the synthesis of an ATP-dependent maltose translocation system. However, MP and b-PGM activity were not affected by a disruption of the MalR-encoding gene. Instead, synthesis of b-PGM has been shown to be exposed to carbon catabolite repression, which was also shown to be the case for MP. pgmB is located in the putative trehalose operon including the genes presumed to code for the trehalose-specific components of the phosphotransferase system transporting trehalose into the cells. Furthermore, directly upstream of pgmB, trePP was localised. This gene was shown to encode a novel phosphorylase, trehalose 6-phosphate phosphorylase (TrePP), catalysing the reversible Pi-dependent phosphorolysis of trehalose 6-phosphate to beta-glucose 1-phosphate and glucose 6-phosphate. TrePP was biochemically characterised and shown to be present in a few other species, mainly Enterococcus faecalis, of low G+C content LAB. The role of b-PGM in trehalose metabolism and in polysaccharide synthesis was assessed by disrupting its encoding gene. b-PGM was shown to be crucial for trehalose assimilation in L. lactis, while the b-PGM-deficient strain continued to grow with a tenfold decreased growth rate on maltose, compared to the wild-type strain. The b-PGM-deficient strain showed an enhanced production of polysaccharide, composed of alpha-1,4-linked glucose units when cultivated on maltose. It was suggested that this polysaccharide was a result from another metabolic pathway, resembling the maltodextrin system in Escherichia coli. Global regulatory circuits play a notable role when cells are adapting to environmental changes. A L. lactis mutant, TMB5003, was obtained by an unknown genetic event in the wild-type strain 19435. TMB5003 possessed an enhanced growth rate in glucose batch culture and a 1.5 times higher specific lactate productivity under non-limiting glucose conditions, compared to 19435. TMB5003 transported glucose by a non-saturating mode and the lactate dehydrogenase activity was twenty times higher in this strain compared to 19435. In conclusion, the results obtained for TMB5003, together with the characterisation of the initial disaccharide metabolism in L. lactis, may lead to improved industrial lactate production from LAB.
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