Division of Infectious Disease, Far Eastern Memorial Hospital, New Taipei City, Taiwan.
Department of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan.
Front Cell Infect Microbiol. 2024 Mar 26;14:1380976. doi: 10.3389/fcimb.2024.1380976. eCollection 2024.
The hemin acquisition system is composed of an outer membrane TonB-dependent transporter that internalizes hemin into the periplasm, periplasmic hemin-binding proteins to shuttle hemin, an inner membrane transporter that transports hemin into the cytoplasm, and cytoplasmic heme oxygenase to release iron. Fur and HemP are two known regulators involved in the regulation of hemin acquisition. The hemin acquisition system of is poorly understood, with the exception of HemA as a TonB-dependent transporter for hemin uptake.
Putative candidates responsible for hemin acquisition were selected via a homolog search and a whole-genome survey of . Operon verification was performed by reverse transcription-polymerase chain reaction. The involvement of candidate genes in hemin acquisition was assessed using an in-frame deletion mutant construct and iron utilization assays. The transcript levels of candidate genes were determined using quantitative polymerase chain reaction.
and operons were selected as candidates for hemin acquisition. Compared with the parental strain, and mutants displayed a defect in their ability to use hemin as the sole iron source for growth. However, hemin utilization by the and mutants was comparable to that of the parental strain. expression was repressed by Fur in iron-replete conditions and derepressed in iron-depleted conditions. HemP negatively regulated expression. Like , was repressed by Fur in iron-replete conditions; however, was moderately derepressed in response to iron-depleted stress and fully derepressed when hemin was present. Unlike and , the operon was constitutively expressed, regardless of the iron level or the presence of hemin, and Fur and HemP had no influence on its expression.
HemA, HemU, and TonB1 contribute to hemin acquisition in . Fur represses the expression of and in iron-replete conditions. expression is regulated by low iron levels, and HemP acts as a negative regulator of this regulatory circuit. expression is regulated by low iron and hemin levels in a -dependent manner.
血红素获取系统由一个外膜 TonB 依赖性转运体组成，该转运体将血红素内化到周质中，周质中的血红素结合蛋白将血红素穿梭，内膜转运体将血红素转运到细胞质中，细胞质中的血红素加氧酶将铁释放出来。Fur 和 HemP 是两个已知的参与血红素获取调节的调节剂。除了 HemA 作为血红素摄取的 TonB 依赖性转运体外，对 的血红素获取系统知之甚少。
通过同源搜索和 的全基因组调查选择负责血红素获取的假定候选物。通过反转录-聚合酶链反应进行操纵子验证。通过框内缺失突变构建体和铁利用测定评估候选基因在血红素获取中的作用。使用定量聚合酶链反应测定候选基因的转录水平。
选择 和 操纵子作为血红素获取的候选物。与亲本菌株相比， 和 突变体在利用血红素作为唯一铁源生长方面存在缺陷。然而， 和 突变体对血红素的利用与亲本菌株相当。在铁充足的条件下，Fur 抑制 的表达，在缺铁的条件下，Fur 解除抑制。HemP 负调控 的表达。与 一样，在铁充足的条件下， 受 Fur 抑制；然而，在缺铁胁迫下适度去抑制，当存在血红素时完全去抑制。与 和 不同， 操纵子无论铁水平或血红素的存在与否都持续表达，Fur 和 HemP 对其表达没有影响。
HemA、HemU 和 TonB1 有助于 在血红素获取中。Fur 在铁充足的条件下抑制 和 的表达。 的表达受低铁水平调节，HemP 作为该调控回路的负调节剂。 的表达受低铁和血红素水平的依赖调节。
