Identification of a two-component signal transduction system from Corynebacterium diphtheriae that activates gene expression in response to the presence of heme and hemoglobin

Corynebacterium diphtheriae, the causative agent of diphtheria, utilizes va rious 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 contr olled under a dual regulatory mechanism in which the diphtheria toxin repre ssor protein (DtxR) and iron repress expression while either heme or hemogl obin is needed to activate transcription. In this study, two clones isolate d from a C. diphtheriae chromosomal library were shown to activate transcri ption from the hmuO promoter in Escherichia coli, Sequence analysis reveale d that these activator clones each carried distinct genes whose products ha d significant homology to response regulators of two-component signal trans duction systems. Located upstream from each of these response regulator hom ologs are partial open reading frames that are predicted to encode the C-te rminal portions of sensor kinases. The full-length sensor kinase gene for e ach of these systems was cloned from the C. diphtheriae chromosome, and con structs each carrying one complete sensor kinase gene and its cognate respo nse regulator were constructed. One of these constructs, pTSB20, which carr ied the response regulator (chrA) and its cognate sensor kinase (chrS), mas shown to strongly activate transcription from the hmuO promoter in a heme- dependent manner in E. coil. A mutation in chrA (chrAD50N), which changed a conserved aspartic acid residue at position 50, the presumed site of phosp horylation by ChrS, to an asparagine, abolished heme-dependent activation. These findings suggest that the sensor kinase ChrS is invoiced in the detec tion of heme and the transduction of this signal, via a phosphotransfer mec hanism, 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-re sponsive mechanism.