Wang Xing-Guo, Scagliotti Joanna P, Hu Linden T
Tupper Research Institute, Division of Geographic Medicine and Infectious Diseases, New England Medical Center, Tufts University School of Medicine, Box 41, 750 Washington Street, Boston, MA 02111, USA.
Microbiology (Reading). 2004 Feb;150(Pt 2):391-397. doi: 10.1099/mic.0.26752-0.
Phospholipids are an important component of bacterial membranes. Borrelia burgdorferi differs from many other bacteria in that it contains only two major membrane phospholipids: phosphatidylglycerol (PG) and phosphatidylcholine (PC). B. burgdorferi appears to lack enzymes required for synthesis of PC through the well-described methylation pathway. However, B. burgdorferi does contain a gene (BB0249) with significant identity to a recently described phosphatidylcholine synthase gene (pcs) of Sinorhizobium meliloti. B. burgdorferi also contains a gene (BB0721) with significant identity to the gene (pgs) encoding phosphatidylglycerolphosphate synthase, an enzyme in the synthetic pathway of PG. Activity of BB0249 was confirmed by cloning the gene into Escherichia coli, which does not produce PC. Transformation with a plasmid carrying BB0249 resulted in production of PC by E. coli, but only in the presence of exogenously supplied choline, as would be predicted for a Pcs. Because loss of Pgs activity is lethal to E. coli, activity of BB0721 was confirmed by the ability of BB0721 to complement an E. coli Pgs(-) mutant. A plasmid containing BB0721 was transformed into a Pgs(-) mutant of E. coli containing a copy of the native gene on a temperature-regulated plasmid. The temperature-regulated plasmid was exchanged for a plasmid containing BB0721 and it was shown that BB0721 was able to replace the lost Pgs function and restore bacterial growth. This study has established the existence and function of two critical enzymes in the synthesis of PC and PG in B. burgdorferi. Understanding of the biosynthetic pathways of PC and PG in B. burgdorferi is the first step in delineating the role of these phospholipids in the pathogenesis of Lyme disease.
磷脂是细菌细胞膜的重要组成部分。伯氏疏螺旋体与许多其他细菌不同，它仅含有两种主要的膜磷脂：磷脂酰甘油（PG）和磷脂酰胆碱（PC）。伯氏疏螺旋体似乎缺乏通过已充分描述的甲基化途径合成PC所需的酶。然而，伯氏疏螺旋体确实含有一个基因（BB0249），与最近描述的苜蓿中华根瘤菌的磷脂酰胆碱合酶基因（pcs）具有显著的同源性。伯氏疏螺旋体还含有一个基因（BB0721），与编码磷脂酰甘油磷酸合酶的基因（pgs）具有显著的同源性，该酶是PG合成途径中的一种酶。通过将该基因克隆到不产生PC的大肠杆菌中，证实了BB0249的活性。用携带BB0249的质粒转化大肠杆菌，结果大肠杆菌产生了PC，但前提是有外源提供的胆碱，这正如对磷脂酰胆碱合酶的预期。由于Pgs活性的丧失对大肠杆菌是致命的，通过BB0721能够互补大肠杆菌Pgs（-）突变体的能力，证实了BB0721的活性。将含有BB0721的质粒转化到含有温度调节质粒上的天然基因拷贝的大肠杆菌Pgs（-）突变体中。将温度调节质粒换成含有BB0721的质粒，结果表明BB0721能够取代丧失的Pgs功能并恢复细菌生长。这项研究证实了伯氏疏螺旋体中PC和PG合成过程中两种关键酶的存在和功能。了解伯氏疏螺旋体中PC和PG的生物合成途径是阐明这些磷脂在莱姆病发病机制中作用的第一步。
