Regulatory Mechanism for Expression of Genes for Iron Uptake in Pseudomonas aeruginosa

在铁缺乏的情况下，铜绿假单胞菌分泌铁载体(吡咯菌素)来螯合和吸收铁。大多数吡咯菌素生物合成(PVD)基因位于染色体上一个103 kb的区域，称为PVD区，该区域还编码FpvA和PvdS，FpvA是一种外膜蛋白，是Ferri-pyoverdin复合体的受体，PvdS是PVD操纵子转录所必需的激活子。这些操纵子的转录在富铁的条件下被铁调节子的全局抑制因子Fur进一步抑制。序列分析和Fur滴定分析表明，PvdS的3个启动子区域不存在与Fur结合的共同序列Fur box，而pVDS的启动子区域存在Fur box。这些结果有力地支持了在富铁条件下没有PVD操纵子的转录是由于Fur介导的pvdS基因的转录抑制。PvdS突变株产生的碱性蛋白酶是野生型菌株的四分之一。这表明PvdS在碱性蛋白水解酶的产生过程中起着重要的作用，尽管不是必需的。使用Tn1722的定点分辨系统分离了在染色体PVD区域具有大而明确的缺失的各种突变体。这种完全缺乏PVD区的突变体在最低限度的培养基中正常生长，表明PVD区不携带任何必需或营养缺陷性基因。构建了保留和缺失fpvA基因的缺失突变株。前者具有摄取外加的Ferri-pyoverdin复合体的能力，而不是后者，这意味着在铜绿假单胞菌中似乎不存在FpvA以外的第二个低亲和力受体。

Under iron limitation, Pseudomonas aeruginosa secretes siderophore (pyoverdin) to chelate and uptake the ferric iron. Most of the pyoverdin biosynthetic (pvd) genes are located in a chromosomal 103-kb region, termed as pvd region, and this region also encodes FpvA,an outer membrane protein that serves as a receptor for ferri-pyoverdin complex, and PvdS,an activator required for transcription of the pvd operons. Transcription of these operons is further repressed under iron-rich conditions by Fur, a global repressor of the iron regulon. Sequence analysis and Fur titration assay demonstrated that the Fur box, a consensus sequence for binding of Fur, is absent in the three pvd promoter regions but present at the promoter region of pvdS.Transcription of pvdS was repressed and depressed under iron-rich and -limiting conditions, respectively. These results strongly support that no transcription of the pvd operons under iron-rich conditions is due to the Fur-mediated transcriptional repression of the pvdS gene. The pvdS mutant produced alkaline proteinase one-fourth in amount of that the wild type strain produced. This indicates that PvdS plays an important, albeit not essential, role in production of alkaline proteinase. Various mutants having large and defined deletion in the chromosomal pvd region were isolated using a site-specific resolution system of Tn1722. Such a mutant completely lacking the pvd region grew normally in minimal media, demonstrating that the pvd region does not carry any essential or auxotrophic genes. The deletion mutanats were constructed that retained and lacked the fpvA gene. The former mutanat, but not the latter one, had the ability to uptake the externally added ferri-pyoverdin complex, implying that a second low-affinity receptor other than FpvA appears not to exist in P.aeruginosa.