Schmitt M P
Laboratory of Bacterial Toxins, Division of Bacterial Products, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, Maryland 20892, USA.
J Bacteriol. 1999 Sep;181(17):5330-40. doi: 10.1128/JB.181.17.5330-5340.1999.
Corynebacterium diphtheriae, the causative agent of diphtheria, utilizes various host compounds to acquire iron. The C. diphtheriae hmuO gene encodes a heme oxygenase that is involved in the utilization of heme and hemoglobin as iron sources. Transcription of the hmuO gene in C. diphtheriae is controlled under a dual regulatory mechanism in which the diphtheria toxin repressor protein (DtxR) and iron repress expression while either heme or hemoglobin is needed to activate transcription. In this study, two clones isolated from a C. diphtheriae chromosomal library were shown to activate transcription from the hmuO promoter in Escherichia coli. Sequence analysis revealed that these activator clones each carried distinct genes whose products had significant homology to response regulators of two-component signal transduction systems. Located upstream from each of these response regulator homologs are partial open reading frames that are predicted to encode the C-terminal portions of sensor kinases. The full-length sensor kinase gene for each of these systems was cloned from the C. diphtheriae chromosome, and constructs each carrying one complete sensor kinase gene and its cognate response regulator were constructed. One of these constructs, pTSB20, which carried the response regulator (chrA) and its cognate sensor kinase (chrS), was shown to strongly activate transcription from the hmuO promoter in a heme-dependent manner in E. coli. A mutation in chrA (chrAD50N), which changed a conserved aspartic acid residue at position 50, the presumed site of phosphorylation by ChrS, to an asparagine, abolished heme-dependent activation. These findings suggest that the sensor kinase ChrS is involved in the detection of heme and the transduction of this signal, via a phosphotransfer mechanism, to the response regulator ChrA, which then activates transcription of the hmuO promoter. This is the first report of a bacterial two-component signal transduction system that controls gene expression through a heme-responsive mechanism.
白喉棒状杆菌是白喉的病原体，它利用多种宿主化合物来获取铁。白喉棒状杆菌的hmuO基因编码一种血红素加氧酶，该酶参与将血红素和血红蛋白作为铁源的利用过程。白喉棒状杆菌中hmuO基因的转录受双重调控机制控制，其中白喉毒素阻遏蛋白（DtxR）和铁会抑制表达，而血红素或血红蛋白则需要激活转录。在本研究中，从白喉棒状杆菌染色体文库中分离出的两个克隆在大肠杆菌中被证明可激活hmuO启动子的转录。序列分析表明，这些激活克隆各自携带不同的基因，其产物与双组分信号转导系统的应答调节因子具有显著同源性。在这些应答调节因子同源物的每一个上游都有部分开放阅读框，预计可编码传感激酶的C末端部分。从白喉棒状杆菌染色体中克隆了这些系统中每一个的全长传感激酶基因，并构建了各自携带一个完整传感激酶基因及其同源应答调节因子的构建体。其中一个构建体pTSB20携带应答调节因子（chrA）及其同源传感激酶（chrS），在大肠杆菌中被证明以血红素依赖的方式强烈激活hmuO启动子的转录。chrA中的一个突变（chrAD50N）将第50位保守的天冬氨酸残基（推测为ChrS磷酸化位点）变为天冬酰胺，消除了血红素依赖的激活作用。这些发现表明，传感激酶ChrS参与血红素的检测，并通过磷酸转移机制将该信号转导至应答调节因子ChrA，然后ChrA激活hmuO启动子的转录。这是关于细菌双组分信号转导系统通过血红素应答机制控制基因表达的首次报道。
