Functional relevance of C. trachomatis trp operon polymorphism - PROJECT SUMMARY The trp operon in several bacterial species is regulated at multiple levels that include transcription, translation, post-translation, and enzyme activity. The recent discovery of a distinct attenuation mechanism of C. trachomatis trp operon, which we termed “trans”-attenuation because it involved a trans-acting factor, the tryptophan- biosensing YtgR repressor, points to the significance of maintaining stringent regulation of expression of the tryptophan salvage genes. Regulation of trp operon by the TrpR repressor and attenuator is thought to be a check on the relatively high metabolic cost of tryptophan biosynthesis, with transcription only induced when tryptophan levels are low. The biosynthesis of tryptophan is an energetically costly process due to its requirement for D-erythrose-4-phosphate, phosphoenolpyruvate and ATP for chorismate biosynthesis, as well as glutamine, phosphoribosyl pyrophosphate and serine for the conversion of chorismate to tryptophan. It is not clear if this applies to C. trachomatis, which only requires one step – serine and indole condensation reaction to form tryptophan. Instead, we propose an alternative model, whereby dysregulated transcription of the tryptophan synthase genes trpB and trpA leads to serine overutilization, diverting this amino acid away from other serine- requiring metabolic pathways, such as phosphatidylserine (PS) biosynthesis. The additional YtgR-mediated regulation of trpBA expression ensures equitable distribution of serine between tryptophan and PS production. The iron-dependent regulation by YtgR is also proposed to be essential for optimal expression of trpBA in tissue environment that support indole-producing microbiome, such as the stratified squamous epithelium of the female lower genital tract. During epithelial cell differentiation, the gene for transferrin receptor, TFRC is progressively downregulated. We previously reported that the slow transferrin receptor recycling pathway is how C. trachomatis acquires iron. Therefore, TFRC downregulation would subject the pathogen to gradual iron starvation. Noteworthy is the location of the microbiome to the uppermost differentiated layers of the squamous epithelium; and YtgR through its iron-responsiveness could be how Chlamydia senses its proximity to indole- producing microbiota. We propose that this underpins YtgR/IGR importance to C. trachomatis. The trans-attenuation function of YtgR on the trp operon relies on a triple tryptophan motif (WWW) that renders translation of the precursor protein YtgCR sensitive to tryptophan-limiting growth conditions. We obtained evidence that there are several genes that are transcriptionally induced when tryptophan is at low levels, but are not regulated by the tryptophan-responsive TrpR repressor. Therefore, there must be another transcriptional regulator responsible. We propose that YtgR fulfills this role, which would mean a more global role for this repressor in regulating transcriptional adaptation strategies in times of tryptophan limitation.
