Liles M R, Viswanathan V K, Cianciotto N P
Department of Microbiology-Immunology, Northwestern University, Chicago, Illinois 60611, USA.
Infect Immun. 1998 Apr;66(4):1776-82. doi: 10.1128/IAI.66.4.1776-1782.1998.
Previously, we had isolated by transposon mutagenesis a Legionella pneumophila mutant that appeared defective for intracellular iron acquisition. While sequencing in the proximity of the mini-Tn10 insertion, we found a locus that had a predicted protein product with strong similarity to PilB from Pseudomonas aeruginosa. PilB is a component of the type II secretory pathway, which is required for the assembly of type IV pili. Consequently, the locus was cloned and sequenced. Within this 4-kb region were three genes that appeared to be organized in an operon and encoded homologs of P. aeruginosa PilB, PilC, and PilD, proteins essential for pilus production and type II protein secretion. Northern blot analysis identified a transcript large enough to include all three genes and showed a substantial increase in expression of this operon when L. pneumophila was grown at 30 degrees C as opposed to 37 degrees C. The latter observation was then correlated with an increase in piliation when bacteria were grown at the lower temperature. Southern hybridization analysis indicated that the pilB locus was conserved within L. pneumophila serogroups and other Legionella species. These data represent the first isolation of type II secretory genes from an intracellular parasite and indicate that the legionellae express temperature-regulated type IV pili.
此前，我们通过转座子诱变分离出一株嗜肺军团菌突变体，该突变体在细胞内获取铁的过程中似乎存在缺陷。在对mini-Tn10插入位点附近进行测序时，我们发现了一个位点，其预测的蛋白质产物与铜绿假单胞菌的PilB具有高度相似性。PilB是II型分泌途径的一个组成部分，IV型菌毛的组装需要该途径。因此，对该位点进行了克隆和测序。在这个4kb的区域内有三个基因，它们似乎以操纵子的形式组织在一起，编码铜绿假单胞菌PilB、PilC和PilD的同源物，这些蛋白质是菌毛产生和II型蛋白质分泌所必需的。Northern印迹分析鉴定出一个足够大的转录本，包含所有三个基因，并显示当嗜肺军团菌在30℃而非37℃下生长时，该操纵子的表达显著增加。后一观察结果与细菌在较低温度下生长时菌毛形成的增加相关。Southern杂交分析表明，pilB位点在嗜肺军团菌血清群和其他军团菌物种中是保守的。这些数据代表了首次从细胞内寄生虫中分离出II型分泌基因，并表明军团菌表达温度调节的IV型菌毛。
