Svensson M, Lohmeier-Vogel E, Waak E, Svensson U, Rådström P
Applied Microbiology, Lund Institute of Technology, Lund University, SE-221 00 Lund, Sweden.
Int J Food Microbiol. 2007 Jan 25;113(2):195-200. doi: 10.1016/j.ijfoodmicro.2006.06.032. Epub 2006 Sep 22.
Exopolysaccharide (EPS)-producing Streptococcus thermophilus strains have attracted interest recently, since the EPSs act as natural viscosifiers and texture enhancers of fermented foods. We have previously reported that the low level of EPS production by S. thermophilus LY03 could be improved by altering the activities of enzymes in the central carbon metabolism involved in the nucleotide sugar metabolism. In this study, we observed a reduced growth in milk for the strains with increased UDP-glucose pyrophosphorylase (GalU) activity together with either enhanced phosphoglucomutase activity, and/or enhanced activity of the Leloir enzymes. Rapid growth of these mutants in milk could be restored by the addition of four specific amino acids, i.e. Glu, His, Met, and Val. This amino acid requirement was confirmed in a defined medium. Furthermore, the 31P NMR spectra showed higher levels of the GalU reactants pyrophosphate (PPi) and UDP-glucose in the engineered strain, TMB 6013, compared to the parent strain, LY03. These products plus Glu and the GalU reactant UTP are known to be involved in the nitrogen regulatory system in many bacteria. Thus, these results suggest that the reaction catalyzed by GalU is connected to the nitrogen demand of these engineered strains.
产胞外多糖（EPS）的嗜热链球菌菌株近来引起了人们的关注，因为胞外多糖可作为发酵食品的天然增稠剂和质构增强剂。我们之前报道过，通过改变参与核苷酸糖代谢的中心碳代谢中酶的活性，可以提高嗜热链球菌LY03的低水平EPS产量。在本研究中，我们观察到，对于UDP-葡萄糖焦磷酸化酶（GalU）活性增加，同时磷酸葡萄糖变位酶活性增强和/或Leloir酶活性增强的菌株，其在牛奶中的生长有所减少。通过添加四种特定氨基酸，即Glu、His、Met和Val，可以恢复这些突变体在牛奶中的快速生长。在限定培养基中证实了这种氨基酸需求。此外，与亲本菌株LY03相比，31P NMR光谱显示工程菌株TMB 6013中GalU反应物焦磷酸（PPi）和UDP-葡萄糖的水平更高。已知这些产物加上Glu和GalU反应物UTP参与许多细菌的氮调节系统。因此，这些结果表明GalU催化的反应与这些工程菌株的氮需求有关。
