LpxT is an inner membrane protein that transfers a phosphate group from the essential lipid undecaprenyl pyrophosphate (C-55PP) to the lipid A moiety of lipopolysaccharide, generating a lipid A tris-phosphorylated species. The protein is encoded by the non-essential lpxT gene, which is conserved in distantly related Gram-negative bacteria. In this work, we investigated the phenotypic effect of lpxT ectopic expression from a plasmid in Escherichia coli. We found that lpxT induction inhibited cell division and led to the formation of elongated cells, mostly with absent or altered septa. Moreover, the cells became sensitive to detergents and to hypo-osmotic shock, indicating that they had cell envelope defects. These effects were not due to lipid A hyperphosphorylation or C-55PP sequestering, but most likely to defective lipopolysaccharide transport. Indeed, lpxT overexpression in mutants lacking the L,D-transpeptidase LdtD and LdtE, which protect cells with outer membrane defects from osmotic lysis, caused cell envelope defects. Moreover, we found that pyrophosphorylated lipid A was also produced in a lpxT deletion mutant, indicating that LpxT is not the only protein able to perform such lipid A modification in E. coli.

中文翻译：

---

大肠杆菌中 lpxT 基因的过度表达会抑制细胞分裂并导致包膜缺陷，但不会改变脂质 A 的整体磷酸化水平。


LpxT 是一种内膜蛋白，它将磷酸基团从必需脂质十一异戊二烯焦磷酸 (C-55PP) 转移到脂多糖的脂质 A 部分，产生脂质 A三磷酸化物质。该蛋白质的编码是 非必需lpxT基因，在远缘相关的革兰氏阴性细菌中保守。在这项工作中，我们研究了大肠杆菌中质粒的lpxT异位表达的表型效应。我们发现lpxT诱导抑制细胞分裂并导致细长细胞的形成，大多数细胞隔膜缺失或改变。此外，细胞对去污剂和低渗休克变得敏感，表明它们存在细胞包膜缺陷。这些影响并非由于脂质 A 过度磷酸化或 C-55PP 隔离所致，而很可能是由于脂多糖转运缺陷所致。事实上，在缺乏 L,D-转肽酶 LdtD 和 LdtE 的突变体中lpxT过表达会导致细胞包膜缺陷，L,D-转肽酶 LdtD 和 LdtE 可以保护具有外膜缺陷的细胞免遭渗透裂解。此外，我们发现在lpxT缺失突变体中也产生了焦磷酸化脂质A，这表明LpxT并不是唯一能够在大肠杆菌中进行这种脂质A修饰的蛋白质。