Ize Bérengère, Stanley Nicola R, Buchanan Grant, Palmer Tracy
Department of Molecular Microbiology, John Innes Centre, Norwich NR4 7UH, UK.
Mol Microbiol. 2003 Jun;48(5):1183-93. doi: 10.1046/j.1365-2958.2003.03504.x.
The Escherichia coli Tat system serves to export folded proteins harbouring an N-terminal twin-arginine signal peptide across the cytoplasmic membrane. Previous work has demonstrated that strains mutated in genes encoding essential Tat pathway components are highly defective in the integrity of their cell envelope. Here, we report the isolation, by transposon mutagenesis, of tat mutant strains that have their outer membrane integrity restored. This outer membrane repair of the tat mutant arises as a result of upregulation of the amiB gene, which encodes a cell wall amidase. Overexpression of the genes encoding the two additional amidases, amiA and amiC, does not compensate for the outer membrane defect of the tatC strain. Analysis of the amiA and amiC coding sequences indicates that the proteins may be synthesized with plausible twin-arginine signal sequences, and we demonstrate that they are translocated to the periplasm by the Tat pathway. A Tat+ strain that has mislocalized AmiA and AmiC proteins because of deletion of their signal peptides displays an identical defective cell envelope phenotype. The presence of genes encoding amidases with twin-arginine signal sequences in the genomes of other Gram-negative bacteria suggests that a similar cell envelope defect may be a common feature of tat mutant strains.
大肠杆菌的双精氨酸转运（Tat）系统用于将携带N端双精氨酸信号肽的折叠蛋白转运穿过细胞质膜。先前的研究表明，编码Tat途径必需组分的基因突变菌株在其细胞壁完整性方面存在高度缺陷。在此，我们报告通过转座子诱变分离出了外膜完整性得以恢复的tat突变菌株。tat突变体的这种外膜修复是由于编码细胞壁酰胺酶的amiB基因上调所致。编码另外两种酰胺酶amiA和amiC的基因过表达并不能弥补tatC菌株的外膜缺陷。对amiA和amiC编码序列的分析表明，这些蛋白可能是带有合理双精氨酸信号序列合成的，并且我们证明它们通过Tat途径转运到周质空间。由于信号肽缺失而使AmiA和AmiC蛋白定位错误的Tat+菌株表现出相同的细胞壁缺陷表型。其他革兰氏阴性细菌基因组中存在编码带有双精氨酸信号序列酰胺酶的基因，这表明类似的细胞壁缺陷可能是tat突变菌株的共同特征。
